Converging element, optical head, and apparatus and method of optically recording and reproducing information

ABSTRACT

An optical head reproduces optical disks of different disk plate thickness thicknesses t1 (0.6 mm) or t2 (1.2 mm) by using light beams of two wavelengths and one object lens. An converging element comprises a central portion and outer portion, wherein the central portion has optimum design plate thickness of 0.6*t1 to t1 and the outer portion has optimum design plate thickness of 0.6 mm. By providing a step difference in the converging element, information can be recorded or reproduced for an information medium of disk plate thickness t1 and for an information medium of disk plate thickness t2, in a state having small side lobes. Alternatively, a step difference is provided in the object lens, and optical distance L2 from a second light source to a condensing optical system is set to 80 to 95% of optical distance L1 from a first light source to the condensing optical system. Alternatively, only light of first wavelength is shielded or diffracted in a ring-like shape.

TECHNICAL FIELD

The invention relates to an optical head and an apparatus for opticallyrecording and reproducing information to and from an optical informationrecording medium.

BACKGROUND ART

An optical memory technology which uses an optical disk having pit-likepattern as a storage medium of high density and high capacity have beenexpanding its applications as a digital audio disk, a video disk, adocument file disk, a data file, and the like. In the optical memorytechnology, information is recorded and reproduced to and from anoptical disk with a light beam narrowed to a very small size with highprecision and high reliability. The recording and reproducing operationmainly depends on the optical system thereof.

An optical head is a main component in the optical system, and its basicfunctions are divided into convergence for forming a very small lightspot diffraction limit, focus and tracking control the optical system,and detection of pit signals. These functions are realized by combiningvarious optical systems and various detection techniques withphotoelectric conversion, according to an object and a use thereof.

An object lens used in an optical head is designed by considering aplate thickness of an optical disk. Its convergence performance isdeteriorated due to spherical aberration for an optical disk having athickness different from the design value, and this makes recording andreproduction difficult. Previously, a compact disc (hereinafter referredto as CD), a video disk, a magneto-optical disk and the like all havinga plate thickness of 1.2 mm, and one optical head can be used forrecording and reproduction for these various optical disks.

Recently, an optical disk of high density and high capacity, called aDVD (digital video disk), have been used practically, and it ishigh-lighted as an information medium which can handle a large amount ofinformation such as a dynamic image. The DVD has smaller pit size in aninformation recording plane in order to increase recording density,relative to the prior art optical disk, CD. Therefore, for an opticalhead used for recording and reproduction of a DVD, a wavelength of lightsource and numerical aperture (hereinafter referred to also as NA) ofconverging lens which determine the spot size are different from thecounterparts for CD. In order to increase recording density, the DVDadopts a large numerical aperture of an object lens. When numericalaperture is increased, optical resolution is improved and recordingdensity is increased. On the other hand, the converged light spot hascoma aberration caused by inclination of the optical disk. Then, inorder to decrease the influence of coma aberration even when thenumerical aperture of the object lens is increased, the thickness of theplate of the optical disk, DVD, is decreased to 0.6 mm. However, whenthe thickness of the plate of the optical disk is decreased, an objectlens used for the optical disk cannot be used for a prior art disk, andthe compatibility between the DVD and the prior art disk cannot berealized.

It is to be noted that for the CD, the wavelength of light source isabout 0.78 pm and NA is about 0.45, while for the DVD, the wavelength oflight source is about 0.63 to 0.65 dun and NA is about 0.6. Therefore,when two types of optical disks, CD and DVD, are recorded or reproducedby a single optical disk drive, an optical head needs two opticalsystems. On the other hand, there is a tendency to use a common opticalsystem for the CD and for the DVD in order to make the drive compact,small and less expensive. For example, a single light source for the DVDis used while two converging lenses for the CD and for the DVD are used,or even for the converging lens, only one converging lens is usedcommonly while the numerical aperture thereof is changed between thatfor the CD and for the DVD mechanically or optically.

In an example of an optical system of an optical head in a drivecompatible with the CD and the DVD, an object lens of numerical aperture0.6 is used as the converging lens. In the object lens, a centralportion of numerical aperture equal to or smaller than 0.37 is designedto make the aberration minimum when light is converged through atransparent flat plate of thickness 0.9 mm, while an outer portion ofnumerical aperture equal to or larger than 0.37 is designed to make theaberration minimum when light is converged through a transparent flatplate of thickness 0.6 mm. A light beam of wavelength 650 nm emitted bya laser diode is collimated by a condenser lens to become a collimatedlight beam, and it is incident on the object lens. When a DVD isreproduced, the light beam narrowed by the object lens forms a lightspot on an information plane in a DVD of thickness 0.6 mm, while itforms a light spot on an information plane of CD in a plate of thickness1.2 mm. Next, the light reflected from the optical disk is condensedagain by the object lens and is detected by a photodetector. Thephotodetector is constructed such that a focus control signal isdetected by an astigmatism technique and that a tracking control signalis detected by a phase difference or push-pull technique.

By using the optical head, when a CD is reproduced, the light beamtransmitting the central portion of the object lens is reflected by themedium plane and enters the photodetector, while the light beamtransmitting the central portion is diverged due to large sphericalaberration and does not enter substantially onto the photo-receivingplane of the photodetector. Thus, the numerical aperture is limitedsubstantially to 0.37. On the other hand, when a DVD is reproduced, thelight beam transmitting the central portion is synthesized with thattransmitting the outer portion to form an light spot, due to smallspherical aberration. All of the reflected light thereof enterssubstantially the photodetector, and reproduction is performed withnumerical aperture 0.6.

However, because the prior art optical head obtains the compatibilitybetween a CD and a DVD by using a light source of wavelength 650 nm, ithas a problem that sufficient signals cannot be obtained from an opticaldisk having wavelength dependence due to difference in reflectivity.This is evident, for example, for a CD-R standardized as a rewritableCD. In the standard of CD-R, the reflectivity is defined to be 65% orhigher in wavelength range of 775 to 820 nm, but it decreases atwavelengths outside the above range and the absorptivity increases. Forexample, the reflectivity decreases to ⅛ times and the absorptivityincreases to 8 times, so that reproduction is impossible and even thedata recorded by optical absorption are erased.

In order to solve this problem on the compatibility between a CD and aDVD, it is proposed to use two light sources of wavelengths 780 and 650nm and to divide the object lens into a central portion and an outerportion surrounding the central portion, wherein the optimum designplate thickness of plate in the central portion is set to 0.9 mm andthat in the outer portion is set to 0.6 mm. However, this techniquecannot be used practically because the spherical aberration becomes toolarge when a DVD is reproduced. In a CD drive, the numerical aperture ofthe object lens is 0.45 for wavelength 650 nm, whereas in the aboveproposal, the numerical aperture is decreased to 0.37 because the lightof lower wavelength 650 nm is used for reproduction of CD. If thenumerical aperture at the central portion of the object lens is about0.37 in the above example, aberration for CD reproduction is about 40 mλ(rms) and that for DVD reproduction is about 30 mλ (rms), so thatreproduction performance is normal. However, when a CD is reproducedwith the light source of wavelength 780 nm, the numerical aperture hasto be about the same as in a conventional CD drive, and the numericalaperture of the central portion of the object lens is 0.45. However,when the central portion of the object lens having the optimum designplate thickness 0.9 mm is enlarged, aberration becomes larger when a DVDis reproduced. If the central portion is enlarged to numerical aperture0.45, aberration increases to 80 ml˜(rms) or higher though it depends ondesign conditions, and sufficient reproduction performance cannot beprovided. An optical head which uses light beams of wavelengths 780 and650 nm and an object lens having double optimum design platethicknesses, as explained above, has not yet been provided forreproducing both a CD and a DVD.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a converging element havingdesired converging performance for at least two types of opticalinformation media by using a plurality of light beams, and an opticalhead and an optical information recording and reproducing apparatuswhich uses the converging element.

A converging element according to the invention is a converging elementwhich converges a light beam from a light source onto each of at leasttwo types of optical information recording media made of a transparentplate having a thickness different between them. The converging elementhas an inner region near a center axis of the light beam and an outerregion far from the center axis. The outer region has a plane optimizedto converge the light beam transmitting the outer region onto a firstoptical information recording medium among the optical informationrecording media, and the inner region has a plane optimized to convergethe light beam transmitting the inner region onto another opticalinformation recording medium having a larger thickness than the firstone. A phase of the light beam transmitting an innermost portion in theplane of the outer region is shifted relative to that of the light beamtransmitting an outermost portion of the plane of the inner region. Anoptical head or an optical information recording and reproducingapparatus according to the invention uses the converging element.According to the structure of the converging element, reproductionperformance for an information recording medium having a thin platethickness (for example DVD) is ensured, while the numerical aperture forreproduction of an information recording medium having a thick plate(for example CD) is increased. Because the numerical aperture forreproduction of an information recording medium having a thick plate canbe increased, reproduction becomes possible with a light source of longwavelength for the information recording medium having a thick plate.For example, an optical head or an information recording and reproducingapparatus can be provided even for an information recording medium suchas CD-R wherein reproduction becomes impossible because the reflectivityis decreased at a wavelength for reproduction of an informationrecording medium having a thin plate thickness such as DVD.

For example, the converging element is an object lens which comprisesthe inner region and the outer region.

Alternatively, for example, the converging element comprises a lenswhich converges the light beam from the light source onto an opticalinformation recording medium and an optical plate element to becooperate therewith. The lens comprises a first inner region near thecenter axis of the light beam and a first outer region far from thecenter axis. The first outer region has a plane optimized to convergethe light beam transmitting the first outer region onto the firstoptical information recording medium, and the first inner region has aplane optimized to converge the light beam transmitting the first innerregion onto the another optical information recording medium having alarger thickness than the first one. The optical plate element comprisesa second inner region and a second outer region divided from the secondinner region with an optical step. The second inner region and the outerregion are arranged such that the light beam transmitting the firstouter region transmits the second outer region while the light beamtransmitting the first inner region transmits the second inner regionwhen the optical plate element is cooperated with the lens.

Alternatively, for example, the element comprises a lens which convergesthe light beam from the light source onto an optical informationrecording medium and an optical plate element to be cooperatedtherewith. The lens comprises an inner region near the center axis ofthe light beam and an outer region far from the center axis. The outerregion has the plane optimized to converge the light beam transmittingthe outer region onto the first optical information recording medium,and the inner region having the plane optimized to converge the lightbeam transmitting the inner region onto the another optical informationrecording medium having a larger thickness than the first one. Theoptical plate element comprises an inner portion and an outer portiondivided from the inner one with an optical step. The inner and outerportions are arranged in combination with the lens such that the lightbeam transmitting the outer region transmits the outer portion and thelight beam transmitting the inner region transmits the inner portion.

Another optical head according to the invention converges a light beamfrom a light source onto each of first and second optical informationrecording media having different thicknesses. It comprises the lightsource which generates a light beam to be converged on the first opticalinformation recording medium and another light beam to be converged onthe second optical information recording medium. A converging elementcomprises an inner region near a center axis of the light beam and anouter region far from the center axis. The outer region having a planeoptimized to converge the light beam transmitting the outer region ontoa first optical information recording medium among the opticalinformation recording media, and the inner region has a plane optimizedto converge the light beam transmitting the inner region onto anotheroptical information recording medium having a larger thickness than thefirst one. A phase of the light beam transmitting an innermost portionin the plane of the outer region is shifted relative to that of thelight beam transmitting an outermost portion of the plane of the innerregion. A photodetector receives a light reflected from the each of theoptical information recording media to convert it to an electric signal.Distance L1 from a first one of the light sources to the convergingelement and distance L2 from a second one of the light sources to theconverging element satisfy a following relationship:L1*0.8<L2<L1*0.95.

A still further optical head according to the invention comprises alight source which generates light beams of second and thirdwavelengths, a converging element comprising a central region havingnumerical aperture of NA1 and an outer region having numerical aperturebetween NA1 and NA2, the outer region being formed to decreaseaberration when light is converged through a transparent flat plate ofthickness of t1, the central region being formed to decrease aberrationwhen light is converged through a transparent flat plate of thicknessbetween t2 and t2*0.7, and an optical system which converges the lightbeam of the first wavelength through the converging element onto a firstinformation plane of the first optical information recording medium ofthickness t1, converges the light beam of the second wavelength throughthe converging element onto a second information plane of the secondoptical information recording medium of thickness t2 larger than t1 andguides the light beam reflected from the first or second informationplane to a photodetector. An optical element is provided in the opticalsystem, and the optical element prevents incidence of the reflectedlight of the first wavelength in a ring belt region in correspondence tonumerical aperture of the converging element between NA1 and NA1*0.7.

As explained above, by shielding or diffracting light at a part of theobject lens having two optimum design plate thicknesses, recording andreproduction becomes possible with the two light sources for informationmedium having different thicknesses, and even an optical disk havingwavelength dependence can be reproduced. Then, the compatibility of allCD disks and DVD disks can be ensured.

An advantage of the invention is to provide a converging element or anoptical head wherein reproduction of CD is possible when NA is increasedfor reproduction of CD and a laser of 780 nm is used while a DVD can bereproduced.

Another advantage of the invention is that compatibility of a DVD and aCD is realized with an optical head of a simple structure using oneconverging element. Thus, the optical head can be fabricated in acompact size, and an optical disk drive can also be fabricated in acompact size.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a diagram of an optical system of an optical head according afirst embodiment of the invention.

FIG. 2 is another diagram of the an optical system of optical headaccording the first embodiment of the invention.

FIG. 3 is a diagram of an object lens in the optical system of anoptical head according the first embodiment of the invention.

FIG. 4 is another diagram of the object lens in the optical system of anoptical head according the first embodiment of the invention.

FIG. 5 is a graph on a relationship between step height in the objectlens and spherical aberration of light spot.

FIG. 6 is a graph on a relationship between step height in the objectlens and side lobes.

FIG. 7 is a schematic diagram of an apparatus of recording andreproducing optical information.

FIG. 8 is a diagram of an optical system of an optical head according asecond embodiment of the invention.

FIG. 9 is another diagram of the optical system of an optical headaccording the second embodiment of the invention.

FIG. 10 is a diagram of a polarizing hologram.

FIG. 11 is a diagram of an object lens according the second embodimentof the invention.

FIG. 12 is another diagram of the object lens according the secondembodiment of the invention.

FIG. 13 is a diagram of an optical system of an optical head according athird embodiment of the invention.

FIG. 14 is another diagram of the optical system of an optical headaccording the third embodiment of the invention.

FIG. 15 is a diagram of an object lens in the optical system of anoptical head according the third embodiment of the invention.

FIG. 16 is another diagram of the object lens in the optical system ofan optical head according the third embodiment of the invention.

FIG. 17 is a diagram of an optical system of an optical head according afourth embodiment of the invention.

FIG. 18 is another diagram of the optical system of an optical headaccording the fourth embodiment of the invention.

FIG. 19 is a diagram of an object lens and a phase shift element in theoptical system of an optical head according the fourth embodiment of theinvention.

FIG. 20 is another diagram of the object lens and the phase shiftelement in the optical system of an optical head according the fourthembodiment of the invention.

FIG. 21 is a diagram of an optical system according a fifth embodimentof the invention.

FIG. 22 is another diagram of the optical system according the fifthembodiment of the invention.

FIG. 23 is a diagram of a structure around an object lens and a phaseshift element according the fifth embodiment of the invention.

FIG. 24 is another diagram of the structure around the object lens andthe phase shift element according the fifth embodiment of the invention.

FIG. 25 is a diagram of an optical system according a sixth embodimentof the invention.

FIG. 26 is another diagram of the optical system according the sixthembodiment of the invention.

FIG. 27 is a diagram of a structure around an optical plate element andan object lens according the sixth embodiment of the invention.

FIG. 28 is another diagram of the structure around the optical plateelement and the object lens according the sixth embodiment of theinvention.

FIG. 29 is a diagram of an optical system of an optical head according aseventh embodiment of the invention.

FIG. 30 is another diagram of the optical system of an optical headaccording the seventh embodiment of the invention.

FIG. 31 is a diagram of an object lens in the optical system of anoptical head.

FIG. 32 is another diagram of the object lens in the optical system ofoptical head.

FIG. 33 is a diagram of an optical system of an optical head accordingan eighth embodiment of the invention.

FIG. 34 is another diagram of the optical system of an optical headaccording the eighth embodiment of the invention.

FIG. 35 is a diagram of an object lens in the optical system of anoptical head according the eighth embodiment of the invention.

FIG. 36 is another diagram of the object lens in the optical system ofan optical head according the eighth embodiment of the invention.

FIG. 37 is a graph on a relationship between focus offset and L2/L1.

FIG. 38 is a diagram for illustrating recording and reproduction for anoptical disk having small thickness such as a DVD by using an opticalsystem of optical head according to a ninth embodiment of the invention.

FIG. 39 is a diagram for illustrating recording and reproduction for anoptical disk having large thickness such as a CD, similarly to FIG. 38.

FIG. 40 is a front view of a light-shielding filter arranged in theoptical system.

FIG. 41 is a graph of a transmittance characteristic of alight-shielding portion in the filter shown in FIG. 40.

FIG. 42 is another graph of a transmittance characteristic of alight-shielding portion in the filter shown in FIG. 40.

FIG. 43 is a diagram for illustrating formation of a light spot by usingan object lens and the lightshielding filter for an optical disk havinga small plate thickness.

FIG. 44 is a diagram for illustrating formation of a light spot by usingan object lens and the lightshielding filter for an optical disk havinga large plate thickness.

FIG. 45 is a diagram for illustrating recording and reproduction for anoptical disk having a small thickness such as a DVD by using an opticalsystem of an optical head according to a tenth embodiment of theinvention.

FIG. 46 is a diagram for illustrating recording and reproduction for anoptical disk having a large thickness such as a CD, similarly to FIG.45.

FIG. 47 is a diagram for illustrating formation of a light spot by usingan object lens and the polarizing hologram for an optical disk having asmall plate thickness.

FIG. 48 is a diagram for illustrating formation of light spot by usingan object lens and the polarizing hologram for an optical disk having alarge plate thickness.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the several views,embodiments of the invention are explained.

A first embodiment of the invention is explained with reference torelevant drawings. FIGS. 1 and 2 show an optical system of optical headaccording this embodiment. FIG. 1 shows a situation for recording andreproduction to and from an optical disk 10 of plate thickness 0.6 mm,while FIG. 2 shows a situation for recording and reproduction to andfrom an optical disk 18 of plate thickness 1.2 mm. In FIG. 1, a lightbeam 2 of wavelength 650 nm is emitted by a laser diode 1, and abouthalf thereof transmits a beam splitter 3 to enter a wavelength filter 4.The wavelength filter 4 is designed to transmit light of wavelength 650nm and to reflect light of wavelength 780 nm. Then, the light beam 2transmits the wavelength filter 4 and is collimated by a condenser lens5 to become a generally collimated light beam. The collimated light beam2 is reflected by a mirror 6, transmits a light-shielding filter 7 andenters an object lens 8 of numerical aperture 0.6. The object lens 8 isdesigned so that a central portion 8 a of numerical aperture equal to orsmaller than 0.45 has minimum aberration for a disk plate thickness 0.9mm while an outer portion 8 b of numerical aperture equal to or largerthan 0.45 has minimum aberration for a disk plate thickness 0.6 mm. Thelight beam 2 is converged by the object lens 8 to form a light spot 11on an information plane in the optical disk 10 of plate thickness 0.6mm.

The light 12 reflected by the optical disk 10 is condensed by the objectlens 8, passes the light-shielding filter 7 and the mirror 6 and iscondensed by the condenser lens 5. Then, the light beam 12 transmits thewavelength filter 4 to enter the beam splitter 3. About half of thelight incident on the beam splitter 3 is reflected. Then, it transmits acylindrical lens 13 and is received by a photodetector 14. Thephotodetector 14 detects not only reproduction signals, but also a focuscontrol signal for making the object lens 8 follow the information planewith an astigmatism technique and a tracking control signal for trackingan information track with a phase difference technique or a push-pulltechnique.

On the other hand, in FIG. 2, a light beam 16 of wavelength 780 nm isemitted by a laser diode 15, and about half thereof transmits a beamsplitter 17 to enter the wavelength filter 4. The wavelength filter 4 isdesigned to reflect light of wavelength 780 nm. Thus, the light beam 16is reflected by the wavelength filter 4 and is collimated by thecondenser lens 5. The collimated light beam 16 passes the mirror 6 andenters the object lens 8. The light beam 16 of wavelength 780 nm isconverged by the object lens 8 to form a light spot 19 on an informationplane in the optical disk 18 of plate thickness 1.2 mm.

Next, the light 20 reflected by the optical disk 18 is collected by theobject lens 8, passes the mirror 6 and is condensed by the condenserlens 5. Then, it is reflected by the wavelength filter to enter the beamsplitter 17. About half of the light incident on the beam splitter 17 isreflected. Then, it transmits a cylindrical lens 21 and is received by aphotodetector 22. The photodetector 22 detects not only reproductionsignals, but also the focus control signal with an astigmatism techniqueand the tracking control signal with a phase difference technique or apush-pull technique.

In the above-mentioned structure using two wavelengths 650 and 780 nm,when a CD is reproduced with light of wavelength 780 nm, the numericalaperture of the central portion 8 a of the object lens 8 has to bedecreased to about 0.45. However, if the numerical aperture of optimumdesign plate thickness 0.9 mm becomes 0.45, the light spot 11 forrecording and reproduction of DVD generates aberration larger than 80 mλrms. Usually a light spot having aberration larger than 80 mλ rms haslarge so-called side lobes, so that recording and reproductionperformance is deteriorated. Therefore, if the light source of 780 nm isadded and only the numerical aperture of the central portion 8 a isincreased in the prior art optical head, the performance is notsatisfactory. In this embodiment, the light source of 780 nm is addedand the numerical aperture of the central portion 8 a is increased.Further, as shown in FIG. 3, a step (difference in level) is provided ata boundary between the outer portion and the inner portion of the objectlens 8 to decrease Zernike's fifth spherical aberration component in theaberration components. Thus, the side lobes of the light spot 11 arereduced and this improves the recording and reproduction performance.

FIGS. 5 and 6 show graphs on a relationship between step height (ordifference in height, converted to difference in phase of light ofwavelength 650 nm) in the object lens and spherical aberration ofconverging spot and on a relationship between step height in the objectlens and side lobes (wherein the main lobe is displayed to haveamplitude of 100%), respectively. It is apparent that by setting anappropriate value of the phase step, the fifth spherical aberration canbe decreased and that the side lobes can be reduced. In order tosuppress the fifth aberration equal to or smaller than 20 mλ (rms), itis preferable that the phase shift is in a range between 50 and 150degrees. When the phase step (phase difference) is changed, the totalaberration is not changed much. In this embodiment, the step is set toan amount in correspondence to 100 degrees of phase difference.

On the other hand, when the optical disk 18 of plate thickness 1.2 mmsuch as CD is used for recording or reproduction, the range of numericalaperture of 0.45 of the object lens 8 is set for the optimum designplate thickness 0.9 mm, so that the aberration of the light transmittingit is suppressed to a similar order to the prior art structure. As shownin FIG. 3, the light beam transmitting the outer portion 8 b of theobject lens 8 has large spherical aberration and diffuses in arelatively wide range in the information plane in an optical disk 18,and the reflected light also is diffused with large sphericalaberration. Therefore, the reflected light of the light transmitting theouter portion 8 b does not enter the photodetector 22 generally. Then,without providing a means for limiting numerical aperture, CDreproduction becomes possible at numerical number 0.45.

FIG. 7 shows schematically an apparatus of recording and reproducingoptical information which uses the above-mentioned optical head 100. Thestructure of the apparatus except the optical head is similar to a priorart one. An optical disk 102 as an information medium is rotated by amotor 102. The optical head 100 is moved along a shaft in a radialdirection. In order to record or reproduce information, in the opticalhead 100 a light beam emitted by a laser diode is focused by an objectlens onto an information recording plane in the optical disk 102. Afocus control signal for making the object lens follow the plane of theoptical disk 102 and a tracking control signal for tracking aninformation track in the optical disk 102 are detected based on outputsignals of the photodetector 22 in the optical head 100. A headcontroller 108 performs focus control and servo control on the opticalhead based on the control signals. Further, a signal processor 110discriminates a type of an optical disk, and records information to theoptical disk 102 with the optical head and reproduces opticalinformation recorded in information tracks in the optical disk 102according to the output signals of the photodetector in the optical head100.

It is to be noted that various optical heads which will be explainedbelow in following embodiments can also be used in the apparatus ofrecording and reproducing optical information.

Next, a second embodiment of the invention is 25 explained withreference to relevant drawings. FIGS. 8 and 9 show an optical system ofan optical head according the second embodiment of the invention. FIG. 8shows a situation for recording and reproduction to and from an opticaldisk 10 of plate thickness 0.6 mm, while FIG. 9 shows a situation forrecording and reproduction to and from an optical disk 18 of platethickness 1.2 mm. In FIG. 8, a first module 31 for DVD comprises a laserdiode 31 a of wavelength 650 nm which is integrated as one body withphotodetectors 31 b and 31 c for receiving light reflected from theoptical disk 10. A light beam 32 of wavelength 650 nm emitted by thelaser diode 31 a in the first module 31 transmits a cover glass 31 d toenter a wavelength filter 33. The wavelength filter 33 transmits lightof 650 nm and reflects light of wavelength 780 nm. Thus, the light, beam32 transmits the wavelength filter 33 and is collimated by a condenserlens 34 to become a generally collimated light beam. The collimatedlight beam 32 transmits a polarizing hologram 35 and a wavelength plate36 to enter an object lens of numerical aperture 0.6. The polarizinghologram 35 and the wavelength plate 36 are integrated as one body, andthey are fixed to a holder 38 with the object lens 37.

As shown in FIG. 10, the polarizing hologram 35 is fabricated by forminga hologram in a LiNb plate made of a birefringence material with protonexchange. It transmits extraordinary light and diffracts ordinary light.The light beam 32 is handled as extraordinary light by the polarizinghologram 35 and it transmits the polarizing hologram 35 withoutdiffraction. The wavelength plate 36 converts light of wavelength 650 nmfrom linear polarization to generally circular polarization and does notchange polarization for light of wavelength 780 nm. Thus, the light beam32 is converted to circular polarization.

The object lens 37 is designed similarly to the counterpart 8 in thefirst embodiment. A central portion 37 a of numerical aperture equal toor smaller than 0.45 has minimum aberration for a disk plate thickness0.9 mm while an outer portion 37 b of numerical aperture equal to orlarger than 0.45 has minimum aberration for a disk plate thickness 0.6mm. The light beam 32 is converged by the object lens 37 to form a lightspot 17 on an information plane in the optical disk 10 of platethickness 0.6 mm.

Next, the light 40 reflected by the optical disk 10 is condensed by theobject lens 37, is converted by the wavelength plate 36 from thecircular polarization to linear polarization having a polarizationdirection perpendicular to a polarization plane of the light beam 32 andenters the polarizing hologram 35. Because the reflected light 40 entersthe polarizing hologram 35 as ordinary light, it is diffracted. Thediffraction divides the reflected light 40 into diffracted light 42 afor detecting focus signal and diffracted light 42 b for detectingtracking signal. The diffracted lights 42 a and 42 b are narrowed by thecondenser lens 34 and are received by the photodetectors 31 b and 31 c,respectively. Reproduction signals are detected by one or both of thephotodetectors. Further, the photodetector 31 b detects a focus controlsignal for making the object lens 37 follow the information plane withspot size detection technique and the photodetector 31 c detects atracking control signal for tracking an information track with phasedifference technique or push-pull technique.

On the other hand, a second module 43 for CD comprises a laser diode 43a of wavelength 780 nm, a hologram 43 d for separating reflected lightfrom an optical disk to give spacial change and photodetectors 43 a, 43b for detecting the reflected light, and they are integrated as onebody. In FIG. 9, a part of a light beam 44 of wavelength 780 nm emittedby the laser diode 43 a in the second module 43 transmits the hologram43 d and enters the wavelength filter 33. Because the wavelength filter33 transmits light of 650 nm and reflects light of wavelength 780 nm,the light beam 44 is reflected by the wavelength filter 33 and iscollimated by the condenser lens 34. The collimated light beam 44transmits the polarizing hologram 35 and the wavelength plate 36 toenter the object lens of numerical aperture 0.6. The light beam 44 ishandled as extraordinary light by the polarizing hologram 35 and ittransmits the polarizing hologram 35 without diffraction. Because thewavelength plate 36 does not convert polarization direction of light ofwavelength 780 nm, the polarization plane of the light beam 44 ismaintained. Thus, the light beam 44 is focused by the object lens 37 toform a light spot 35 on an information plane in the optical disk 18 ofplate thickness 1.2 mm.

The light 46 reflected by the optical disk 18 is condensed by the objectlens 37, transmits the wavelength plate 36 and the polarizing hologram35. Because the wavelength plate 36 does not change polarizationdirection for light of wavelength 780 mm, the reflected light 46transmits the wavelength plate 36 as linear polarization, similarly tothe light beam 44. Because the reflected light 46 enters the polarizinghologram 35 as extraordinary light, it is not diffracted. The light 46transmitting the polarizing hologram 35 is narrowed by the condenserlens 34 and is reflected by the wavelength filter 33 to enter the secondmodule 43. The reflected light 46 entering the second module 43 isdiffracted by the hologram 43 d to enter the photodetectors 43 b and 43c, and reproduction signals are detected by one or both of thephotodetectors. Further, the photodetector 43 b detects a focus controlsignal for making the object lens 37 follow the information plane with aspot size detection technique and the photodetector 43 c detects atracking control signal for tracking an information track with a phasedifference technique or a push-pull technique.

In the above-mentioned structure using two wavelengths 650 and 780 nm,when a CD is reproduced with light of wavelength 780 nm, the numericalaperture of the central portion 37 a of the object lens 37 has to bedecreased to about 0.45. However, if the numerical aperture of optimumdesign plate thickness 0.9 mm becomes 0.45, the light spot 11 forrecording and reproduction of DVD generates aberration larger than 80 mλrms. Usually a light spot having aberration larger than 80 mλ rms haslarge so-called side lobes, so that recording and reproductionperformance is deteriorated. Therefore, if the light source of 780 nm isadded and only the numerical aperture of the central portion 8 a isincreased in the prior art optical head, the performance is notsatisfactory. In this embodiment, the numerical aperture of the centralportion 37 a is increased, and similarly to the first embodiment, asshown in FIG. 11, a step (difference in level) is provided at a boundarybetween the outer portion and the inner portion of the object lens 37 todecrease fifth spherical aberration component in the aberrationcomponents. Thus, the side lobes of the light spot 39 are reduced toimprove the recording and reproduction performance.

In order to suppress the fifth aberration to 20 mλ (rms) or less, it isfound that it is desirable that the phase shift has a value in a rangebetween 50 and 150 degrees. It is also found that the total aberrationis not changed much when the phase step (phase difference) is changed.In this embodiment, the step is provided by forming a smooth curve inorder to improve formability of the object lens. By using such a lenshaving a smooth shape, an object lens made of glass can be formed whileensuring stable performance against change in ambient temperature. Onthe other hand, when the optical disk 18 of plate thickness 1.2 mm suchas a CD is used for recording or reproduction, the range of numericalaperture of 0.45 of the object lens 37 is set for the optimum designplate thickness 0.9 mm, so that the aberration of the light transmittingit is suppressed to a similar order to the prior art structure.

As shown in FIG. 12, the light beam transmitting the outer portion 37 bof the object lens 37 has large spherical aberration and diffuses in arelatively wide range in the information plane in an optical disk 18,and the reflected light is also diffused with large sphericalaberration. Therefore, the reflected light of the light transmitting theouter portion 37 b does not enter the photodetectors 43 b, 43 cgenerally. Then, without providing a means for limiting numericalaperture, CD reproduction becomes possible at numerical number 0.45.

It is apparent from the above-mentioned explanation that according tothe first and second embodiments a lens can be provided which canreproduce a CD as well as a DVD by increasing NA for CD reproduction andby using a laser of 780 nm. Thus, compatibility of a DVD and a CD isrealized with a simple optical head including one object lens. Further,an optical head can be fabricated compactly, and an optical disk drivecan also be manufactured compactly.

Next, a third embodiment of the invention is explained with reference torelevant drawings. An optical head of the third embodiment has a simplestructure which realizes compatibility of a DVD and a CD by using oneobject lens having double optimum design plate thicknesses. The objectlens has large NA for CD reproduction and can reproduce a CD with alaser of 780 nm while reproducing a DVD.

FIGS. 13 and 14 shows an optical system of optical head according thethird embodiment of the invention. FIG. 13 shows a situation forrecording and reproduction to and from an optical disk 10 of platethickness 0.6 mm, while FIG. 14 shows a situation for recording andreproduction to and from an optical disk 18 of plate thickness 1.2 mm.An optical system uses laser diodes 1 and 15 which generate light beamsof 650 nm and of 870 nm, respectively, while it uses a common objectlens 108 which focuses the light beam onto an optical disk. In detail, asection consisting of a laser diode, a beam splitter, a cylindrical lensand a photodetector is provided for each wavelength, but light beams ofdifferent wavelengths from two optical paths are guided to one opticalpath by using a wavelength filter which transmits light of wavelength650 nm and reflects light of wavelength 780 nm. A further section fromthe wavelength filter 4 to the object lens 108 is used commonly.Generally, in an optical head for reproduction of optical informationmedia of disk plate thickness t2 (0.6 mm) or t1 (1.2 mm), the optimumdesign plate thickness at the central portion of the object lens 108 isset to a value from t1*0.6 to t1 and that at the outer portion is set to0.6 mm. The range of the optimum design plate thickness is determinedexperimentally. Further, a step is provided for a phase shift element(optical plate element) 107 to be cooperated with the object lens 108.Then, information can be recorded or reproduced for an informationmedium of disk plate thickness t1 and for that of disk plate thicknesst2 in a state where side lobes are small.

The optical head is explained further. When an optical disk 10 of platethickness 0.6 mm is recorded or reproduced with a light beam ofwavelength 650 nm, in FIG. 14, about a half of a light beam 2 (for DVD)of wavelength 650 nm emitted by the first laser diode 1 transmits thebeam splitter 3 to enter the wavelength filter 4. The wavelength filter4 is designed to transmit light of wavelength 650 nm and to reflectlight of wavelength 780 nm. Thus, the light beam 2 transmits thewavelength filter 4 and is collimated by a condenser lens 5 to become agenerally collimated light beam. The collimated light beam 2 isreflected by a mirror 6, transmits a phase shift element 107 as theoptical plate element and enters the object lens 108 of numericalaperture 0.6.

With reference FIGS. 15 and 16, a structure and arrangement of the phaseshift element 107 and the object lens 108 are explained. The phase shiftelement 107 and the object lens 108 are arranged so that the centersthereof coincide with the center axis of the light beam. The object lens108 comprises a central portion (inner region) 108 a near the centeraxis of light beam and an outer portion (outer region) 108 b far fromthe center axis. The central portion 108 a has a plane optimized toconverge the light beam transmitting the inner region onto a thinoptical disk, while the outer portion 108 b has a plane optimized toconverge the light beam transmitting the outer region onto an opticaldisk thicker than the optical disk having the thin thickness. Further,the phase shift element 107 is an optical plate element having a step inorder to change the phase. By combining the phase shift element 107 andthe object lens 108, the phase of the light beam transmitting aninnermost portion of the outer portion 108 b of the object lens 108 isshifted relative to the phase of the light beam transmitting anoutermost portion of the central portion 108 a.

In concrete, the object lens 108 is designed so that a central portion108 a of numerical aperture equal to or smaller than 0.45 has minimumaberration for a disk plate thickness 0.9 mm while an outer portion 108b of numerical aperture equal to or larger than 0.45 has minimumaberration for a disk plate thickness 0.6 mm. The light beam 2 isconverged by the object lens 108 to form a light spot 111 on aninformation plane in the optical disk 10 of plate thickness 0.6 mm.

Next, the light 112 reflected by the optical disk 10 is condensed by theobject lens 108, passes the phase shift element 107, and the mirror 6and is condensed by the condenser lens 5. Then, the narrowed light beam112 transmits the wavelength filter 4 to enter the beam splitter 3.About half of the light incident on the beam splitter 3 is reflected.Then, it transmits a cylindrical lens 13 and is received by aphotodetector 14. The photodetector 14 detects not only reproductionsignals, but also a focus control signal for making the object lens 108follow the information plane with astigmatism technique and a trackingcontrol signal for tracking an information track with phase differencetechnique or push-pull technique.

On the other hand, as shown in FIG. 14, a light beam 16 (for CD) ofwavelength 780 nm is emitted by the laser diode 15, and about halfthereof transmits a beam splitter 17 to enter the wavelength filter 4.Because the wavelength filter 4 is designed to reflect light ofwavelength 780 nm, the light beam 16 is reflected by the wavelengthfilter 4 and is collimated by the condenser lens 5. The collimated lightbeam 16 passes the mirror 6, transmits the phase shift element 107 andenters the object lens 108 of numerical aperture 1.2. The light beam 2of wavelength 780 nm is converged by the object lens 108 to form a lightspot 119 on an information plane in the optical disk 18 of platethickness 1.2 mm.

The light 120 reflected by the optical disk 18 is collected by theobject lens 108, passes the mirror 6 and is condensed by the condenserlens 5. Then, it is reflected by the wavelength filter 4 to enter thebeam splitter 17. About half of the light incident on the beam splitter17 is reflected. Then, it transmits a cylindrical lens 21 and isreceived by a photodetector 22. The photodetector 22 detects not onlyreproduction signals, but also the focus control signal with anastigmatism technique and the tracking control signal with a phasedifference technique or a push-pull technique.

The object lens 108 and the phase shift element 107 are fixed so as tokeep dynamical balance relative to a center of gravity 123 of a movingdevice comprising an object lens holder 109 having a drive means movablein focus direction as optical axis of the object lens and in trackingdirection as a radial direction of the disk. Because the dynamicalbalance of the object lens 108 and the phase shift element 107 is keptrelative to the center of gravity of the movable device, even when abalancer or the like is not used, distortion relative to the opticalaxis of the object lens 108 is small. Therefore, an optical head and aninformation recording and reproducing apparatus have good quality ofsignals on recording and on reproduction.

In the above-mentioned structure using two wavelengths 650 and 780 nm,when a CD is reproduced with light of wavelength 780 nm, the numericalaperture of the central portion 108 a of the object lens 108 has to bedecreased to about 0.45. However, if the numerical aperture of optimumdesign plate thickness 0.9 mm becomes 0.45, the light spot 11 forrecording and reproduction of DVD generates aberration larger than 80 mλrms. Usually a light spot having aberration larger than 80 mλ rms haslarge so-called side lobes, so that recording and reproductionperformance is deteriorated. Therefore, if the light source of 780 nm isadded and only the numerical aperture of the central portion 8 a isincreased in the prior art structure, the performance is notsatisfactory. In this embodiment, the numerical aperture of the centralportion 108 a of the object lens 108 is increased, and an optical stepis provided at a boundary between the inner portion 107 a and the outerportion 107 b of the phase shift element 107. Thus, fifth sphericalaberration component in the aberration components is decreased for thelight spot formed after transmitting the object lens 108. As shown forexample in FIG. 15, the optical step is formed by thinning the thicknessin the inner portion 107 b. By providing the step, the side lobes of thelight spot 39 are reduced for light beams of the two wavelengths toimprove the recording and reproduction performance. Because the phaseshift element 107 and the object lens 108 are provided separately, it isadvantageous that the shape of the object 108 can be simplified. It isalso advantageous that a glass lens favorable for change in ambienttemperature can be adopted easily. In order to suppress the fifthaberration equal to or smaller than 20 mλ (rms), it is preferable thatthe phase shift is in a range between 50 and 150 degrees. When the phasestep is changed, the total aberration is not changed much. In thisembodiment, the step is set to an amount in correspondence to 125degrees of phase difference of light of wavelength 650 nm.

On the other hand, when the optical disk 18 of plate thickness 1.2 mmsuch as CD is used for recording or reproduction, the range of numericalaperture of 0.45 of the object lens 108 is set for the optimum designplate thickness 0.9 mm, so that the aberration of the light transmittingit is suppressed to a similar order to the prior art structure. As shownin FIG. 16, the light beam transmitting the outer portion 108 b of theobject lens 108 has large spherical aberration and is diverged in arelatively wide range in the information plane in an optical disk 18,and the reflected light also is diverged with large sphericalaberration. Therefore, the reflected light of the light transmitting theouter portion 108 b does not enter the photodetector 22 generally. Then,without providing a means for limiting numerical aperture, CDreproduction becomes possible at numerical number 0.45.

Next, an optical head according to a fourth embodiment of the inventionis explained with reference to relevant drawings. An optical head of thefourth embodiment comprises modules 31, 43 wherein a light source and aphotodetector are integrated as one body. Further, a phase shift element137, a wavelength plate 136 and a polarizing hologram 135 are integratedas one body. FIGS. 17 and 18 show an optical system of the optical head.FIG. 17 shows a situation for recording and reproduction to and from anoptical disk 10 of plate thickness 0.6 mm, while FIG. 18 shows asituation for recording and reproduction to and from an optical disk 18of plate thickness 1.2 mm.

In FIG. 17, the first module 31 for DVD has a laser diode 31 a ofwavelength 650 nm and photodetectors 31 b, 31 c for detecting lightreflected from an optical disk 10. The laser diode 31 a and thephotoconductors 31 b, 31 c are integrated as one body. A light beam 32of wavelength 650 nm emitted from the laser diode 31 a transmits a coverglass 31 d to enter a wavelength filter 33. The wavelength filter 33 isdesigned to transmit light of wavelength 650 nm and to reflect light ofwavelength 780 nm. Thus, the light beam 32 transmits the wavelengthfilter 33 and is collimated by a condenser lens 34 to become a generallycollimated light beam. The collimated light beam 32 transmits apolarizing hologram 135, a wavelength plate 136 and a phase shiftelement 137 as an optical plate element and enters the object lens 108of numerical aperture 0.6.

The polarizing hologram 135, the wavelength plate 136 and the phaseshift element 137 are integrated as one body, and they are fixed withthe object lens 138 to a holder 139 for the object lens 138. As shown inFIG. 10, the polarizing hologram 135 is fabricated by forming a hologramin a LiNb plate made of a birefringence material with proton exchange.It transmits extraordinary light and diffracts ordinary light. The lightbeam 32 is handled as extraordinary light by the polarizing hologram135, and it transmits the polarizing hologram 135 without diffraction.The wavelength plate 136 converts light of wavelength 650 nm from linearpolarization to generally circular polarization, while it does notchange polarization for light of wavelength 780 nm. Thus, the light beam32 is converted to circular polarization.

Similarly to the object lens 108 in the third embodiment, the objectlens 38 is designed to have double optimum plate thicknesses. As shownin FIGS. 19 and 20, a central portion 138 a of numerical aperture equalto or smaller than 0.45 is designed to have minimum aberration for adisk plate thickness 0.9 mm while an outer portion 108 b of numericalaperture equal to or larger than 0.45 is designed to have minimumaberration for a disk plate thickness 0.6 mm. The light beam 32 isconverged by the object lens 138 to form a light spot 141 on aninformation plane in the optical disk 10 of plate thickness 0.6 mm.

The light 140 reflected by the optical disk 10 is condensed by theobject lens 138, passes the phase shift element 137 and is converted bythe wavelength plate 136 from circular polarization to linearpolarization in polarization direction perpendicular to polarizationplane of the light beam 32. Because the reflected light 140 enters thepolarizing hologram 135 as ordinary light, it is diffracted thereby. Onthe diffraction, the reflected light is divided to a diffracted light142 a for detecting focus signal and another 142 b for detectingtracking signal. The diffracted lights 142 a, 142 b are narrowed by acondenser lens 34 to enter the photodetectors 31 b, 31 c, andreproduction signals are detected by one or both of the photodetectors.Further, the photodetector 31 b detects a focus control signal formaking the object lens 37 follow the information plane with spot sizedetection technique and the photodetector 31 c detects a trackingcontrol signal for tracking an information track with phase differencetechnique or push-pull technique.

On the other hand, the second module 43 for CD has a laser diode 43 a ofwavelength 780 nm, a hologram 43 d for separating the reflected light togive spacial change and photodetectors 43 b, 43 c for detectingreflected light from an optical disk 10, and the laser diode 43 a, thehologram 43 d and the photoconductors 43 b, 43 c are integrated as onebody. In FIG. 18, a part of a light beam 44 of wavelength 780 nm emittedfrom the laser diode 43 a transmits the hologram 43 d to enter thewavelength filter 33. Because the wavelength filter 33 reflects light ofwavelength 780 nm, the light beam 44 is reflected by the wavelengthfilter 33 and collimated by a condenser lens 34. The collimated lightbeam 44 transmits the polarizing hologram 135, the wavelength plate 136and the phase shift element 137 and enters the object lens of numericalaperture 0.6. The polarizing hologram 32 handles the light beam 44 asextraordinary light, and the light beam 44 transmits it withoutdiffraction. The wavelength plate does not change the polarizationdirection for light of wavelength 780 nm, so that the polarization planeof the light beam 44 is maintained. The light beam 44 is focused by theobject lens 44 and forms a light spot 149 on an information plane in anoptical disk 18 of plate thickness 1.2 mm.

The light 146 reflected by the optical disk 18 is collected by theobject lens 138 and transmits the phase shift element 137, thewavelength plate 136 and the polarizing hologram 135. Because thewavelength plate 136 does not change polarization direction for light ofwavelength 780 nm, the reflected light 146 transmits the wavelengthplate 136 as linear polarization, similarly to the light beam 44.Because the reflected light 146 enters the polarizing hologram 135 asextraordinary light, it is not diffracted. The light 146 transmittingthe polarizing hologram 135 is narrowed by the condenser lens 34 and isreflected by the wavelength filter 33 to enter the second module 43. Thereflected light 46 entering the second module 43 is diffracted by thehologram 43 d to enter the photodetectors 43 b and 43 c, andreproduction signals are detected by one or both of the photodetectors.Further, the photodetector 43 b detects a focus control signal formaking the object lens 37 follow the information plane with a spot sizedetection technique and the photodetector 43 c detects a trackingcontrol signal for tracking an information track with a phase differencetechnique or a push-pull technique. Further, the object lens 138 and thephase shift element 137 are fixed so as to keep dynamical balancerelative to a center of gravity 153 of a moving device comprising theobject lens holder 139 having a drive means movable in focus directionas optical axis of the object lens and in tracking direction as a radialdirection of the disk.

In the above-mentioned structure using two wavelengths 650 and 780 nm,when a CD is reproduced with light of wavelength 780 nm, the numericalaperture of the central portion 138 a of the object lens 138 isincreased similarly to the third embodiment, and as shown in FIG. 11, astep is provided at a boundary between the outer portion and the innerportion of the object lens 137. Thus, the fifth spherical aberrationcomponent in the aberration components of the light spot 140 formedafter transmitting the object lens 138 is decreased, and the side lobesof the light spot 149 are reduced, to improve the recording andreproduction performance. In order to suppress the fifth aberrationequal to or smaller than 20 mλ (rms), it is preferable that the phaseshift is in a range between 50 and 150 degrees. When the phase step ischanged, the total aberration is not changed much. In this embodiment,the step is set to an amount in correspondence to 125 degrees of phasedifference.

On the other hand, when the optical disk 18 of plate thickness 1.2 mmsuch as a CD is used for recording or reproduction, the range ofnumerical aperture of 0.45 of the object lens 138 is set for the optimumdesign plate thickness 0.9 mm, so that the aberration of the lighttransmitting it is suppressed to a similar order to the prior artstructure. As shown in FIG. 20, the light beam transmitting the outerportion 138 b of the object lens 138 has large spherical aberration andis diverged in a relatively wide range in the information plane in anoptical disk 18, and the reflected light also is diverged with largespherical aberration. Therefore, the reflected light of the lighttransmitting the outer portion 138 b does not enter the photodetectors43 a, 43 c generally. Then, without providing a means for limitingnumerical aperture, CD reproduction becomes possible at numerical number0.45.

As explained above, the phase shift element 137 has the optical stepadded to the optical plate element. In the third embodiment, the phaseshift element 137 is provided separately, while in the fourthembodiment, the phase shift element 137 is integrated with thepolarizing hologram and the phase shift element. As to the two cases,the phase shift element having the optical step can be fabricated, forexample, by forming a step with etching or with molding of transparentresin. Alternatively, instead of the step, a similar function can beobtained by depositing an anisotropic film of a different refractiveindex. Alternatively, a step (difference in level) or a film ofdifferent refractive index is formed on one of the planes of thepolarizing hologram. Needless to say, it may also be formed on thewavelength plate.

The phase shift element has a simple structure that the phase of lightbeam is changed by the step or the film of different refractive index,so that it is easy to optimize the phase shift. Therefore, optimizationfor each optical head in various models is easily performed, anddevelopment period can be shortened to a large extent.

In the above-mentioned third and fourth embodiments, two light sourcesare used, and light beams of different wavelengths are used forrecording and reproduction for optical disks of transparent disks ofdifferent thicknesses. However, for an optical head using a light beamof one wavelength, an object lens having an inner region and an outerregion and a phase shift element having an inner region and an outerregion can be used similarly to the above embodiments, so as to improveperformance of recording and reproduction for two types of opticalinformation recording media.

Next, advantages of the third and fourth embodiments are explained. Inthese embodiments, an optical head can be used for CD reproduction aswell as for DVD reproduction, by increasing NA for CD reproduction andby using a laser of 780 nm. The compatibility for a CD and a DVD can berealized with a simple optical head including one optical head. Further,an optical head can be fabricated compactly, and an optical informationrecording and reproducing apparatus can be fabricated compactly andsimply.

Further, the means for shifting phase is realized by a simple techniqueto shift the phase of light beam by forming the step or by depositingthe film of different refractive index, so that it is easy to optimizethe phase shift. Therefore, optimization for each optical head invarious models is easily performed, and development period can beshortened to a large extent.

Because the dynamical balance of the object lens and the phase shiftelement is kept relative to the center of gravity of the movable device,even when a balancer or the like is not used, distortion relative to theoptical axis of the object lens 108 is small. Therefore, an optical headand an information recording and reproducing apparatus have good qualityof signals on recording and on reproduction.

Next, a fifth embodiment of the invention is explained with reference toFIGS. 21 to 24. FIGS. 21 and 22 show an optical system of an opticalhead according the fifth embodiment of the invention. FIG. 21 showssituation for recording and reproduction to and from an optical disk 10of plate thickness 0.6 mm such as a DVD, while FIG. 22 shows a situationfor recording and reproduction to and from an optical disk 18 of platethickness 1.2 mm such as a CD. Further, FIGS. 23 and 24 show a structurearound the object lens and the phase shift element according the fifthembodiment of the invention.

In FIG. 21, a light beam 2 of wavelength 650 nm is emitted by a laserdiode 1, and about half thereof transmits a beam splitter 3 to enter awavelength filter 4. The wavelength filter 4 is designed to transmitlight of wavelength 650 nm and to reflect light of wavelength 780 nm.Then, the light beam 2 transmits the wavelength filter 4 and iscollimated by a condenser lens 5 to become a generally collimated lightbeam. The collimated light beam 2 is reflected by a mirror 6, transmitsan optical plate element 271 and enters an object lens 208 of numericalaperture 0.6. The light beam 2 transmitting the optical plate element271 is converged by the object lens 208 to form a light spot 211 on aninformation plane in the optical disk 10 of plate thickness 0.6 mm. Theoptical plate element 271 and the object lens 208 are held as anintegral body with a holder 209 for holding the object lens, and itsposition is controlled by a driver 23.

The light 212 reflected by the optical disk 10 is condensed by theobject lens 208, passes the optical plate element 271 and the mirror 6and is narrowed by the condenser lens 5. Then, the light beam 212transmits the wavelength filter 4 to enter the beam splitter 3. Abouthalf of the light incident on the beam splitter 3 is reflected. Then, ittransmits a cylindrical lens 13 and is received by a photodetector 14.The photodetector 14 detects not only reproduction signals, but also afocus control signal for making the object lens 208 follow theinformation plane with an astigmatism technique and a tracking controlsignal for tracking an information track with a phase differencetechnique or a push-pull technique.

On the other hand, in FIG. 22, a light beam 16 of wavelength 780 nm isemitted by a laser diode 15, and about half thereof transmits a beamsplitter 17 to enter the wavelength filter 4. Because the wavelengthfilter 4 is designed to reflect light of wavelength 780 nm, the lightbeam 16 is reflected by the wavelength filter 4 and is collimated by thecondenser lens 5. The collimated light beam 16 passes the mirror 6,transmits the optical plate element 271 and enters the object lens 208of numerical aperture 1.2. The light beam 2 of wavelength 780 nm isconverged by the object lens 8 to form a light spot 19 on an informationplane in the optical disk 18 of plate thickness 1.2 mm.

Next, the light 220 reflected by the optical disk 18 is collected by theobject lens 208, transmits the optical plate element 27 1, is reflectedby the mirror 6 and condensed by the condenser lens 5. Then, it isreflected by the wavelength filter 4 to enter the beam splitter 17.About half of the light incident on the beam splitter 17 is reflected.Then, it transmits a cylindrical lens 21 and is received by aphotodetector 22. The photodetector 22 detects not only reproductionsignals, but also the focus control signal with an astigmatism techniqueand the tracking control signal with a phase difference technique or apush-pull technique.

Here, the optical plate element 271 and the object lens 208 areexplained in detail. The object lens 208 is designed so that aberrationbecomes minimum for disk plate thickness 0.6 mm for all the portion ofNA equal to or smaller than 0.6 when only the object lens 208 is usedwithout associated with the optical plate element 271. That is, it has aplane optimized to converge the light beam transmitting the object lens208 onto an optical disk of thin transparent plate. Then, the objectlens 208 can be use for an optical head for recording or reproducing toand from an optical disk of plate thickness 0.6 mm.

On the other hand, the optical plate element 271 has the inner region271 a near the central axis of light beam and the outer region 271 b farfrom the central axis. In an optical head which uses two wavelengths 650and 780 nm, when a disk of plate thickness 0.6 mm is reproduced withlight of wavelength 780 nm, NA of the inner region 271 a of the opticalplate element 271 has to be set to about 0.45. However, when NA ofoptimum design plate thickness 0.9 mm becomes 0.45, aberration exceeding80 mλ rms is generated in the light spot 211 for recording orreproduction of a DVD. Usually a light spot having aberration largerthan 80 mλ rms has large so-called side lobes, so that recording andreproduction performance is deteriorated. Therefore, if the light sourceof 780 nm is added and only the numerical aperture of the centralportion is increased in the prior art structure, the performance is notsatisfactory. In this embodiment, as shown in FIG. 23, the numericalaperture of the inner region 271 a of the optical plate element 271 isincreased, and a step is provided at a boundary between the inner region271 a and the outer region 271 b of the optical plate element 271. Then,Zernike's fifth spherical aberration component in the aberrationcomponents is decreased, and the side lobes of the light spot arereduced, to improve the recording and reproduction performance. In thisembodiment, the amplitude of the step is set to 125 degrees of phasedifference.

The optical plate element 271 is designed to have a plane whichminimizes aberration for a transparent flat plate of disk platethickness 0.9 mm when cooperated with the object lens 271 when it iscooperated with the object lens 208. The plane of the inner region ofthe optical plate element 271 has a plane optimized to converge thelight beam transmitting the inner region 271 a onto an optical disk ofthinner transparent plate among a plurality of optical disks. Such atransparent plate is, for example, a transparent plate having thicknessequal to or larger than t1*0.7 wherein t1 denotes the thickness oftransparent plate having the largest thickness in a plurality of typesof optical disks (1.2 mm in this example). On the other hand, the outerregion 271 b of NA equal to or larger than 0.45 has a flat plane, and itonly gives phase shift determined by a product of the thickness of theouter region 271 b and the refractive index of the optical plate element271. Further, the phase of the outer region is set so that the phase ofthe beam transmitting the innermost portion of the outer region is setto be shifted relative to the phase of the beam transmitting theoutermost portion of the inner region. The optical plate element 271 ismade of glass, and the shape of its surface is obtained by etching thesurface of a flat glass sheet.

A relationship between the above-mentioned step (converted to the phaseof the light of wavelength 650 nm) and the aberrations is similar tothat shown in FIG. 5 on the first embodiment, and values of the step andthe side lobes are similar generally to those in the graph shown in FIG.6. By setting the phase step to an appropriate value, the fifthspherical aberration is decreased, and side lobes can be reduced. Inorder to suppress the fifth aberration below 20 mλ (rms), it is alsofound that it is necessary to set the phase shift between 50 and 150degrees. When the amplitude of the phase step is changed, totalaberration is not affected much.

On the other hand, when an optical disk 18 of plate thickness 1.2 mmsuch as a CD is used for recording or reproduction, the aberration ofthe light beam transmitting the range of NA 0.45 of the optical plateelement 271 is suppressed to a similar degree to that of prior art bysetting the range to optimum design plate thickness 0.9 mm. As shown inFIG. 24, because the light beam transmitting the outer region 271 la ofthe optical plate element 271 transmits the object lens 208 wholly, ithas large aberration and is dispersed in a relatively wide range on theinformation plane in the optical disk 271. Further, the reflected lighttransmitting the outer region 271 b is also dispersed with largespherical aberration. Then, the reflected light transmitting the outerregion 271 b does not enter the photodetector 22 substantially, and adisk of plate thickness 1.2 mm can be reproduced with NA 0.45 withoutproviding a means for limiting the aperture.

Next, a sixth embodiment of the invention is explained with reference toFIGS. 25 to 28. FIGS. 25 and 26 show an optical system of optical headaccording the sixth embodiment of the invention. FIG. 25 shows asituation for recording and reproduction to and from an optical disk ofplate thickness 0.6 mm such as a DVD, while FIG. 26 shows a situationfor recording and reproduction to and from an optical disk of platethickness 1.2 mm such as a CD. Further, FIGS. 27 and 28 show detailsaround the object lens and the phase shift element.

Recording and reproduction to and from an optical 10 disk 18 of platethickness 1.2 mm such as a CD are explained. In FIG. 27, a first module31 for a DVD comprises a laser diode 31 a of wavelength 650 nmintegrated as one body with photodetectors 31 b and 31 c for receivinglight reflected from the optical disk 10. A light beam 32 of wavelength650 nm emitted by the laser diode 31 a in the first module 31 transmitsa cover glass 31 d to enter a wavelength filter 33. The wavelengthfilter 33 transmits light of 650 nm and reflects light of wavelength 780nm. Thus, the light beam 32 transmits the wavelength filter 33 and iscollimated by a condenser lens 34 to become a generally collimated lightbeam. The collimated light beam 32 transmits a polarizing hologram 235and a wavelength plate 235 to enter an object lens 208 of numericalaperture 0.6.

The polarizing hologram 235 and the wavelength plate 236 are integratedas one body, and they are fixed to a holder 209 with the object lens208. As shown in FIG. 10, the polarizing hologram 235 is fabricated byforming a hologram in a LiNb plate made of a birefringence material withproton exchange. It transmits extraordinary light and diffracts ordinarylight. The light beam 32 is handled as extraordinary light by thepolarizing hologram 235 and it transmits the polarizing hologram 235without diffraction. The wavelength plate 236 converts light ofwavelength 650 nm from linear polarization to generally circularpolarization, but it does not change polarization for light ofwavelength 780 nm. Thus, the light beam 32 is converted to circularpolarization by the wavelength plate 236. The light beam of circularpolarization transmits the optical plate element 271 and is converged bythe object lens 208 to form a light spot 241 on an information plate inan optical disk 10 of plate thickness 0.6 mm.

The object lens 208 and the optical plate element 271 are designedsimilarly to the counterparts in the fifth embodiment. The object lens208 is designed so that aberration becomes minimum for the portion of NAequal to or smaller than 0.45 for an optical disk of plate thickness 0.6mm when it is used without cooperated with the optical plate element271. On the other hand, the optical plate element 271 comprises an innerregion 271 a near the optical axis and an outer region 271 b fartherefrom. The numerical aperture of the inner region 271 a isincreased, while a step is provided between the inner and outer regions271 a, 271 b. The amplitude of the step is set to 125 as phase step.

Next, the light 240 reflected by the optical disk 10 is condensed by theobject lens 208, transmits the optical plate element 271, is convertedby the wavelength plate 236 from the circular polarization to linearpolarization having a polarization direction perpendicular to apolarization plane of the light beam 32 and enters the polarizinghologram 235. Because the reflected light 240 enters the polarizinghologram 235 as ordinary light, it is diffracted. The diffractiondivides the reflected light 240 into diffracted light 242 a fordetecting focus signal and diffracted light 242 b for detecting trackingsignal. The diffracted lights 242 a and 242 b are narrowed by thecondenser lens 34 to be received by the photodetectors 31 b and 31 c,respectively, and reproduction signals are detected by one or both ofthe photodetectors. Further, the photodetector 31 b detects a focuscontrol signal for making the object lens 208 follow the informationplane with spot size detection technique and the photodetector 31 cdetects a tracking control signal for tracking an information track withphase difference technique or push-pull technique.

On the other hand, a second module 43 for CD comprises a laser diode 43a of wavelength 780 nm, a hologram 43 d for separating reflected lightfrom an optical disk to give spacial change and photodetectors 43 a, 43b which detects the reflected light, and they are integrated as onebody. In FIG. 25, a part of a light beam 44 of wavelength 780 nm emittedby the laser diode 43 a transmits the hologram 43 d and enters thewavelength filter 33. Because the wavelength filter 33 transmits lightof 650 nm and reflects light of wavelength 780 nm, the light beam 44 isreflected by the wavelength filter 33 and is collimated by the condenserlens 34. The collimated light beam 44 transmits the polarizing hologram235 and the wavelength plate 236 to enter the object lens of numericalaperture 0.6. The light beam 44 is handled as extraordinary light by thepolarizing hologram 235 and it transmits the polarizing hologram 235without diffraction. Because the wavelength plate 236 does not convertpolarization direction of light of wavelength 780 nm, the polarizationplane of the light beam 44 is maintained. Thus, the light beam 44 isfocused by the object lens 208 to form a light spot 249 on aninformation plane in the optical disk 18 of plate thickness 1.2 mm.

Next, the light 246 reflected by the optical disk 25 18 is condensed bythe object lens 208 and transmits the wavelength plate 236 and thepolarizing hologram 235. Because the wavelength plate 236 does notchange polarization direction for light of wavelength 780 mm, thereflected light 246 transmits the wavelength plate 236 as linearpolarization, similarly to the light beam 44. Because the reflectedlight 246 enters the polarizing hologram 235 as extraordinary light, itis not diffracted. The light 246 transmitting the polarizing hologram235 is narrowed by the condenser lens 34 and is reflected by thewavelength filter 33 to enter the second module 43. The reflected light246 entering the second module 43 is diffracted by the hologram 43 d toenter the photodetectors 43 b and 43 c, and reproduction signals aredetected by one or both of the photodetectors. Further, thephotodetector 43 b detects a focus control signal for making the objectlens 208 follow the information plane with a spot size detectiontechnique and the photodetector 43 c detects a tracking control signalfor tracking an information track with a phase difference technique or apush-pull technique.

By setting the phase step to an appropriate value, the fifth sphericalaberration is decreased, and side lobes can also be reduced, similarlyto the fifth embodiment as explained above with reference FIGS. 5 and 6.In order to suppress the fifth aberration below 20 mλ (rms), it is alsofound that it is necessary to set the phase shift between 50 and 150degrees. When the amount of the phase step is changed, total aberrationis not affected much.

On the other hand, when an optical disk 18 of plate thickness 1.2 mmsuch as CD is used for recording or reproduction, the aberration of thelight beam transmitting the range of NA 0.45 of the optical plateelement 271 is suppressed to a similar degree to that of prior art bysetting the range to optimum design plate thickness 0.9 mm. As shown inFIG. 28, because the light beam transmitting the outer region 271 a ofthe optical plate element 271 transmits the object lens 208 wholly, ithas large aberration and is dispersed in a relatively wide range on theinformation plane in the optical disk 271. Further, the reflected lighttransmitting the outer region 271 b is also dispersed with largespherical aberration. Then, the reflected light transmitting the outerregion 271 b does not enter the photodetector 22 substantially, and adisk of plate thickness 1.2 mm can be reproduced with NA 0.45 withoutproviding a means for limiting the aperture.

In the fifth and sixth embodiments, the optical plate element 271 isfabricated by etching a glass plate, but it may also be fabricated byforming glass with a press. Because the lens effect of the optical plateelement 271 is weak, a resin of low refractive index may be used.Therefore, injection molding or press forming for resin may also beadopted similarly.

According to the fifth and sixth embodiments, a necessary function canbe added to an object lens by providing the optical plate element in theoptical path. Further, this is a means for realizing a form difficult tobe added as an object lens, and an optical head or an optical disk drivehaving a desired condensing performance can be provided easily.

Further, because the optical plate element has a simple structurefabricated by etching on a generally plate-like surface, its shape caneasily be optimized. Therefore, optimization or the like for each modelof optical head can easily be performed by using a common object lens,and a period for development can be shortened to a large extent. Theoptical plate element is also appropriate for production in small lots.

Further, because the optical plate element and the object lens areseparate parts, the shape of the object lens can be simplified. A glasslens advantageous for change in ambient temperature can be adoptedeasily.

By adopting the above-mentioned structure, the reproduction performancefor an information recording medium of small thickness such as a DVD canbe kept, while numerical aperture for an information recording medium oflarge thickness such as a DVD can be increased.

Further, because the numerical aperture for an information recordingmedium of large thickness can be increased, the information recordingmedium of large thickness can be reproduced with a light source oflonger wavelength. For example, reproduction becomes possible even foran information recording medium such as CD-R which cannot reproduceinformation due to low reflectance with use of a light source for awavelength for reproduction of an information recording medium ofsmaller thickness.

In the above-mentioned first to sixth embodiments, the range of optimumdesign plate thickness 0.9 mm is set to NA 0.45 as an example forcompatibility between a DVD and a CD. Further, it is possible to extendthe range further to about 0.50 for the compatibility between a DVD anda laser disk (LD).

Next, a seventh embodiment of the invention is explained with referenceto relevant drawings. FIGS. 29 and 30 show an optical system of opticalhead according to the seventh embodiment. FIG. 29 shows a situation forrecording and reproduction to and from an optical disk 10 of platethickness 0.6 mm, while FIG. 30 shows a situation for recording andreproduction to and from an optical disk 18 of plate thickness 1.2 mm.

In FIG. 29, a light beam 2 of wavelength 650 nm is emitted by a laserdiode 1, and about half thereof transmits a beam splitter 3 to enter awavelength filter 4. The wavelength filter 4 is designed to transmitlight of wavelength 650 nm and to reflect light of wavelength 780 nm.Then, the light beam 2 transmits the wavelength filter 4 and iscollimated by a condenser lens 5 to become a generally collimated lightbeam. The collimated light beam 2 is reflected by a mirror 6 and entersan object lens 308 of numerical aperture 0.6. The object lens 308comprises a central portion 308 a and an outer portion 308 b. Thecentral portion 308 a is designed so that a central portion 308 a ofnumerical aperture equal to or smaller than 0.45 has minimum aberrationfor a disk plate thickness 0.9 mm while an outer portion 308 b ofnumerical aperture equal to or larger than 0.45 has minimum aberrationfor a disk plate thickness 0.6 mm. The light beam 2 is converged by theobject lens 308 to form a light spot 311 on an information plane in theoptical disk 10 of plate thickness 0.6 mm.

Next, the light 312 reflected by the optical disk 10 is condensed by theobject lens 308, passes a mirror 6 and is narrowed by a condenser lens5. The narrowed reflected light 312 transmits a wavelength filter 4 toenter a beam splitter 17. About half of the light incident on the beamsplitter 17 is reflected. Then, the reflected light transmits acylindrical lens 13 and is received by a photodetector 14. Thephotodetector 14 detects not only reproduction signals, but also a focuscontrol signal for making the object lens 308 follow the informationplane with an astigmatism technique and a tracking control signal fortracking an information track with a phase difference technique orpush-pull technique.

On the other hand, in FIG. 30, a light beam 16 of wavelength 780 nm isemitted by a laser diode 15, and about half thereof transmits a beamsplitter 17 to enter the wavelength filter 4. The wavelength filter 4 isdesigned to reflect light of wavelength 780 nm. Then, the light beam 16is reflected by the wavelength filter 4 and is collimated by thecondenser lens 5. The collimated light beam 16 passes the mirror 6 andenters the object lens 308. The light beam 16 of wavelength 780 nm isconverged by the object lens 308 to form a light spot 319 on aninformation plane in the optical disk 18 of plate thickness 1.2 mm.

Next, the light 320 reflected by the optical disk 18 is collected by theobject lens 308, passes the mirror 6 and is condensed by the condenserlens 5. Then, it is reflected by the wavelength filter 4 to enter thebeam splitter 17. About half of the light incident on the beam splitter17 is reflected. Then, it transmits a cylindrical lens 21 and isreceived by a photodetector 22. The photodetector 22 detects not onlyreproduction signals, but also the focus control signal with anastigmatism technique and the tracking control signal with a phasedifference technique or a push-pull technique.

In the above-mentioned structure using two wavelengths 650 and 780 nm,when a CD is reproduced with light of wavelength 780 nm, the numericalaperture of the central portion 308 a of the object lens 308 has to bedecreased to about 0.45. However, if the numerical aperture of optimumdesign plate thickness 0.9 mm becomes 0.45, the light spot 11 forrecording and reproduction of DVD generates aberration larger than 80 mλrms. Usually a light spot having aberration larger than 80 mλ rms haslarge so-called side lobes, so that recording and reproductionperformance is deteriorated. Therefore, if the light source of 780 nm isadded and only the numerical aperture of the central portion 308 a isincreased in the prior art structure, the performance is notsatisfactory. Then, in this embodiment, as shown in FIG. 30, a step isprovided at a boundary between the outer and inner portions of theobject lens 308 to decrease Zernike's fifth spherical aberrationcomponent in the aberration components. Thus, the side lobes of thelight spot 311 are reduced to improve the recording and reproductionperformance.

The relationship between step (converted to phase of light of wavelength650 nm) in the object lens and spherical aberration of converging spotand the relationship between step height in the object lens and sidelobe (wherein the main lobe is displayed to have amplitude of 100%) aresimilar to the graphs shown in FIGS. 5 and 6 in the first embodiment. Itis apparent that by setting an appropriate value of the phase step, thefifth spherical aberration can be decreased and the side lobes can bereduced. In order to suppress the fifth aberration equal to or smallerthan 20 mλ (rms), it is preferable that shift of the phase is in a rangebetween 50 and 150 degrees. When the phase is changed, the totalaberration is not changed much. In this embodiment, amplitude of thestep is set to an amount in correspondence to 100 degrees of phasedifference.

On the other hand, when the optical disk 18 of plate thickness 1.2 mmsuch as a CD is used for recording or reproduction, the range ofnumerical aperture of 0.45 of the object lens 308 is set for the optimumdesign plate thickness 0.9 mm, so that the aberration of the lighttransmitting it is suppressed to a similar order to the prior artstructure. However, it is preferable for suppression of aberration thatoptical length L2 from the laser diode 15 to the condenser lens 5 is setto a value between 80 and 95% of optical length L1 from the laser diode1 to the condenser lens 5. Further, as shown in FIG. 32, the light beamtransmitting the outer portion 308 b of the object lens 308 has largespherical aberration and diverges in a relatively wide range in theinformation plane in an optical disk 18, and the reflected light also isdiverged with large spherical aberration. Therefore, the reflected lightof the light transmitting the outer portion 308 b does not enter thephotodetector 22 generally. Then, without providing a means for limitingnumerical aperture, CD reproduction becomes possible at numerical number0.45. If the optical length L2 is set to a value equal to or smallerthan 80% of L1, the degree of diffusion is decreased, and it is notdesirable for reproduction performance of a CD.

Next, an eighth embodiment of the invention is explained with referenceto relevant drawings. FIGS. 33 and 34 show an optical system in theoptical system of an optical head according to the invention. FIG. 33shows a situation for recording and reproduction to and from an opticaldisk 10 of plate thickness 0.6 mm, while FIG. 34 shows a situation forrecording and reproduction to and from an optical disk 18 of platethickness 1.2 mm.

In FIG. 33, a first module 31 for a DVD comprises a laser diode 31 a ofwavelength 650 nm integrated as one body with photodetectors 31 b and 31c for receiving light reflected from the optical disk 10. In the firstmodule 31, a light beam 32 of wavelength 650 nm emitted by the laserdiode 31 a transmits a cover glass 31 d to enter a wavelength filter 33.The wavelength filter 33 transmits light of 650 nm and reflects light ofwavelength 780 nm. Then, the light beam 32 transmits the wavelengthfilter 33 and is collimated by a condenser lens 34 to become a generallycollimated light beam. The collimated light beam 32 transmits apolarizing hologram 335 and a wavelength filter 33 to enter an objectlens of numerical aperture 0.6. The polarizing hologram 335 and thewavelength plate 336 are integrated as one body, and they are fixed to aholder 38 with the object lens 337. As shown in FIG. 10, the polarizinghologram 335 is fabricated by forming a hologram in a LiNb plate made ofa birefringence material with proton exchange. It transmitsextraordinary light and diffracts ordinary light. The light beam 32 ishandled as extraordinary light by the polarizing hologram 335 and ittransmits the polarizing hologram 335 without diffraction. Thewavelength plate 336 converts light of wavelength 650 nm from linearpolarization to generally circular polarization and does not changepolarization for light of wavelength 780 nm. Thus, the light beam 32 isconverted to circular polarization. The object lens 337 is designedsimilarly to the counterpart 308 in the seventh embodiment. As shown inFIGS. 35 and 36, the central portion 337 a with numerical aperture in arange of plate thickness 0.9 mm is designed to have minimum aberrationfor disk plate thickness 0.9 mm. The light beam 32 is converged by theobject lens 337 to form a light spot 339 on an information plane in theoptical disk 10 of plate thickness 0.6 mm. designed to have minimumaberration for disk plate thickness 0.9 mm. The light beam 32 isconverged by the object lens 337 to form a light spot 339 on aninformation plane in the optical disk 10 of plate thickness 0.6 mm.

Next, the light 340 reflected by the optical disk 10 is condensed by theobject lens 337, is converted by the wavelength plate 336 from thecircular polarization to linear polarization having a polarizationdirection perpendicular to a polarization plane of the light beam 32 andenters the polarizing hologram 335. Because the reflected light 340enters the polarizing hologram 335 as ordinary light, it is diffracted.The diffraction divides the reflected light 340 into diffracted light342 a for detecting focus signal and diffracted light 342 b fordetecting tracking signal. The diffracted lights 342 a and 342 b arenarrowed by the condenser lens 334 to be received by the photodetectors31 b and 31 c, respectively, and reproduction signals are detected byone or both of the photodetectors. Further, the photodetector 31 bdetects a focus control signal for making the object lens 337 follow theinformation plane with spot size detection technique and thephotodetector 31 c detects a tracking control signal for tracking aninformation track with phase difference technique or push-pulltechnique.

On the other hand, a second module 43 for CD comprises a laser diode 43a of wavelength 780 nm, a hologram 43 d for separating reflected lightfrom an optical disk to give spacial change and photodetectors 43 a, 43b for detecting the reflected light, and they are integrated as onebody. In FIG. 34, a part of a light beam 44 of wavelength 780 nm emittedby the laser diode 43 a transmits the hologram 43 d and enters thewavelength filter 33. Because the wavelength filter 33 is designed toreflect light of wavelength 780 nm, the light beam 44 is reflected bythe wavelength filter 33 and is collimated by the condenser lens 34. Thecollimated light beam 44 transmits the polarizing hologram 335 and thewavelength plate 336 to enter the object lens of numerical aperture 0.6.The light beam 44 is handled as extraordinary light by the polarizinghologram 335, and it transmits the polarizing hologram 335 withoutdiffraction. The reflected light 346 transmitting the polarizinghologram 335 is narrowed by the condenser lens 34 and is reflected bythe wavelength filter 33 to enter the second module 43. The reflectedlight 46 entering the second module 43 is diffracted by the hologram 43d to enter the photodetectors 43 b and 43 c, and reproduction signalsare detected by one or both of the photodetectors. Further, thephotodetector 43 b detects a focus control signal for making the objectlens 337 follow the information plane with spot size detectiontechnique, and the photodetector 43 c detects a tracking control signalfor tracking an information track with phase difference technique orpush-pull technique.

In the above-mentioned structure using two wavelengths 650 and 780 nm,when a CD is reproduced with light of wavelength 780 nm, the numericalaperture of the central portion 337 a of the object lens 337 has to bedecreased to about 0.45. However, if the numerical aperture of optimumdesign plate thickness 0.9 mm becomes 0.45, the light spot for recordingand reproduction of DVD generates aberration larger than 80 mλ rms.Usually a light spot having aberration larger than 80 mλ rms has largeso-called side lobes, so that recording and reproduction performance isdeteriorated. Therefore, if the light source of 780 nm is added and onlythe numerical aperture of the central portion 308 a is increased in theprior art structure, the performance is not satisfactory. In thisembodiment, the numerical aperture of the central portion 308 a isincreased, and similarly to the seventh embodiment, as shown in FIG. 35,a step is provided at a boundary between the outer portion and the innerportion of the object lens 337 to decrease fifth spherical aberrationcomponent in the aberration components. Thus, the side lobes of thelight spot 339 are reduced to improve the recording and reproductionperformance. In order to suppress the fifth aberration to 20 mλ (rms) orless, it is found that it is desirable that the phase shift has a valuein a range between 50 and 150 degrees. It is also found that the totalaberration is not changed much when the phase step is changed. In thisembodiment, the step is formed with a smooth curve in order to improveformability of the object lens. By using such a lens having a smoothshape, an object lens made of glass can be formed while stableperformance is ensured against change in ambient temperature. Theamplitude of the step is set to a value to be converted to 100 degreesof phase difference.

On the other hand, when the optical disk 18 of plate thickness 1.2 mmsuch as CD is used for recording or reproduction, the range of numericalaperture of 0.45 of the object lens 337 is set for the optimum designplate thickness 0.9 mm, so that the aberration of the light transmittingit is suppressed to a similar order to the prior art structure. However,it is preferable for suppression of aberration that optical length L2from the laser diode 43 a to the condenser lens 34 is set to a valuebetween 80 and 95% of optical length L1 from the laser diode 31 a to thecondenser lens 34. FIG. 37 shows change in focus offset plotted againstL2. In this case, it is found that the focus offset can be set to zeroby setting L2 to 90% of L1. Further, the light beam transmitting theouter portion 337 b of the object lens 337 has large sphericalaberration and diverges in a relatively wide range in the informationplane in an optical disk 18, and the reflected light also is divergedwith large spherical aberration. Therefore, the reflected light of thelight transmitting the outer portion 337 b does not enter thephotodetectors 43 b, 43 c generally. Then, without providing a means forlimiting numerical aperture, CD reproduction becomes possible atnumerical number 0.45. If the optical length L2 is set to a value equalto or smaller than 80% of optical length L1, the degree of diffusion isdecreased, and it is not desirable for reproduction performance of CD.

In the seventh and eighth embodiments, a similar advantage is realizedby using a lens fabricated by integrating a condenser lens and an objectlens as one body.

It is apparent from the above-mentioned explanation that according tothe seventh and eighth embodiments a lens can be provided which canreproduce a CD as well as a DVD by increasing NA for CD reproduction and20 by using a laser of 780 nm. Thus, compatibility of a DVD and a CD isrealized with a simple optical head using one lens. Further, an opticalhead can be fabricated compactly, and an optical disk drive can also bemanufactured compactly.

Further, by shortening the optical length from the laser for a CD to thecondenser lens than that from the laser a for DVD to the condenser lens,aberration is suppressed to improve the quality of reproduction signals,and focus offset is decreased.

In the above-mentioned first to eighth embodiments, the range of optimumdesign plate thickness 0.9 mm is set to NA 0.45 as an example forcompatibility between a DVD and a CD. However, it is possible to set theoptimum design plate thickness to a value equal to or larger than 1.0mm. Further, it is also possible to extend the range further to about0.50 for the compatibility between DVD and laser disk (LD).

Next, a ninth embodiment of the invention is explained with reference torelevant drawings. FIG. 38 shows a situation for recording andreproduction to and from an optical disk 10 of plate thickness 0.6 mm,while FIG. 39 shows a situation for recording and reproduction to andfrom an optical disk 18 of plate thickness 1.2 mm. In FIG. 38, a lightbeam 2 of wavelength 650 nm is emitted by a laser diode 1, and abouthalf thereof transmits a beam splitter 3 to enter a wavelength filter 4.The wavelength filter 4 is designed to transmit light of wavelength 650nm and to reflect light of wavelength 780 nm. Then, the light beam 2transmits the wavelength filter 4 and is collimated by a condenser lens5 to become a generally collimated light beam. The collimated light beam2 is reflected by a mirror 6, transmits a light-shielding filter 407 andenters an object lens 408 of numerical aperture 0.6. The light-shieldingfilter 407 and the object lens 408 are fixed to a holder 409. As shownin FIG. 40, the light-shielding filter 407 comprises a ring-likelight-shielding portion 407 a and a transmitting portion 407 b. As shownin FIG. 41, the light-shielding portion 407 a shields light ofwavelength 650 nm and transmits light of wavelength 780 nm, while asshown in FIG. 42, the transmitting portion 407 b has wavelengthcharacteristic that light is transmitted for both wavelengths of 650 and780 nm. Further, as shown in FIG. 43, the light-shielding portion 407 ashields a part of the light beam in correspondence to numerical aperturefrom 0.37 to 0.45.

The object lens 408 is designed so that a central portion 408 a ofnumerical aperture equal to or smaller than 0.45 has minimum aberrationfor a disk plate thickness 0.9 mm while an outer portion 408 b ofnumerical aperture equal to or larger than 0.45 has minimum aberrationfor a disk plate thickness 0.6 mm. The light beam 2 shielded by thelight-shielding filter 7 as a ring form is converged by the object lens408 to form a light spot 411 on an information plane in the optical disk10 of plate thickness 0.6 mm.

The light 412 reflected by the optical disk 10 is condensed by theobject lens 408, passes the light-shielding filter 407 and the mirror 6and is narrowed by the condenser lens 5. Then, the narrowed light beam412 transmits the wavelength filter 4 to enter the beam splitter 3.About half of the light incident on the beam splitter 3 is reflected.Then, it transmits a cylindrical lens 13 and is received by aphotodetector 14. The photodetector 14 detects not only reproductionsignals, but also a focus control signal for making the object lens 8follow the information plane with a astigmatism technique and a trackingcontrol signal for tracking an information track with a phase differencetechnique or a push-pull technique.

On the other hand, in FIG. 39, a light beam 16 of wavelength 780 nm isemitted by a laser diode 15, and about half thereof transmits a beamsplitter 17 to enter the wavelength filter 4. Because the wavelengthfilter 4 is designed to reflect light of wavelength 780 nm, the lightbeam 16 is reflected by the wavelength filter 4 and is collimated by thecondenser lens 5. The collimated light beam 16 passes the mirror 6 andthe light-shielding filter 407 to enter the object lens 408 of numericalaperture 1.2. As shown in the wavelength characteristics in FIGS. 41 and42, the light beam 16 of wavelength 780 nm transmits both of thelight-shielding portion 407 a and the transmitting portion 407 b and isconverged by the object lens 408 to form a light spot 419 on aninformation plane in the optical disk 18 of plate thickness 1.2 mm.

The light 420 reflected by the optical disk 18 is collected by theobject lens 408, passes the mirror 6 and the light-shielding filter 407and is narrowed by the condenser lens 5. Then, it is reflected by thewavelength filter 4 to enter the beam splitter 17. About half of thelight incident on the beam splitter 17 is reflected. Then, it transmitsa cylindrical lens 21 and is received by a photodetector 22. Thephotodetector 22 detects not only reproduction signals, but also thefocus control signal with an astigmatism technique and the trackingcontrol signal with a phase difference technique or a push-pulltechnique.

In the above-mentioned structure using two wavelengths 650 and 780 nm,when a CD is reproduced with light of wavelength 780 nm, the numericalaperture of the central portion 408 a of the object lens 408 has to bedecreased to about 0.45. However, if the numerical aperture of optimumdesign plate thickness 0.9 mm becomes 0.45, the light spot 411 forrecording and reproduction of DVD generates aberration larger than 80 mλrms. Then, recording and reproduction performance is deteriorated, andif the light source of 780 nm is added and only the numerical apertureof the central portion 408 a is increased in the prior art structure,the performance is not satisfactory. In this embodiment, the numericalaperture of the central portion 408 a is increased, and as shown in FIG.43, the light-shielding portion 407 a of the light-shielding filter 407shields a part of the light beam 2 in correspondence to numericalaperture from 0.37 to 0.45. Thus, aberration is decreased. That is, thelight spot 411 used for recording or reproduction of DVD is formed bysynthesizing a light beam transmitting the central portion 408 a of theobject lens 408 of numerical aperture equal to or smaller than 0.37 andanother light beam transmitting the outer portion 408 b of numericalaperture from 0.45 to 0.6. Aberration is small in a region near thecenter of optical axis of the object lens 408 even when the design platethickness is deviated, and for numerical aperture of about 0.37 theaberration is about 30 mλ rms when the light transmitting the centralportion of plate thickness 0.9 mm is focused on an optical disk of platethickness of 0.6 mm. Then, the recording and reproducing performance isnot deteriorated. Therefore, recording and reproduction can be possiblefor a DVD by using the above-mentioned light spot 411 synthesized fromlight beams transmitting the outer portion 408 b and the central portion408 a of numerical aperture equal to or smaller than 0.37.

When an optical disk of thickness 1.2 mm such as a CD is subjected torecording or reproduction, the range of numerical number 0.45 of theobject lens 408 is designed for optimum design plate thickness 0.9 mm,so that aberration of the light transmitting the object lens can besuppressed to a value about the same as that in a prior art device. Asshown in FIG. 44, the light beam transmitting the outer portion 408 b ofthe object lens 408 has large spherical aberration and it is diverged ina relatively wide range on an information plane in an optical disk 18,and the reflected light thereof is also diverged. Therefore, thereflected light transmitting the outer portion 408 b does not enter thephotodetector 22 generally. Because the light-shielding filter 407 doesnot shield the light beam 16, the numerical aperture of the object lensbecomes 0.45 for recording or reproduction of a CD, and the numericalnumber of the same order as in a conventional CD drive can be obtained.

In this embodiment, the light-shielding filter 407 is a filter using anoptical thin film. Further, a similar advantage can be obtained byproviding a hologram having a function of wavelength selection fordiffracting the light of wavelength 650 nm only for the light-shieldingregion 407 a. In the above-mentioned example, a region of numericalaperture 0.45 of the object lens 408 is provided for optimum designplate thickness 0.9 mm. This region is a practical region becauseaberration on CD reproduction is suppressed to 40 mλ rms for 70% or moreof the plate thickness of optical disk 18. In the above-mentionedexample, the light-shielding portion 407 a corresponds to numericalaperture from 0.45 to 0.37 of the object lens 408. The performance forDVD can be secured for 70% or more of the numerical aperture 0.45 forCD.

Next, a tenth embodiment of the invention is explained with reference torelevant drawings. FIG. 45 shows a situation for recording andreproduction to and from an optical disk 10 of plate thickness 0.6 mm,while FIG. 46 shows a situation for recording and reproduction to andfrom an optical disk 18 of plate thickness 1.2 mm. In FIG. 45, a firstmodule 31 for DVD comprises a laser diode 31 a of wavelength 650 nmwhich is integrated as one body with photodetectors 31 b and 31 c forreceiving light reflected from the optical disk 10. In the first module31 a, light beam 32 of wavelength 650 nm emitted by the laser diode 31 atransmits a cover glass 31 d to enter a wavelength filter 33. Thewavelength filter 33 transmits light of 650 nm and reflects light ofwavelength 780 nm. Then, the light beam 32 transmits the wavelengthfilter 33 and is collimated by a condenser lens 34 to become a generallycollimated light beam. The collimated light beam 32 transmits apolarizing hologram 35 and a wavelength plate 436 to enter an objectlens 437 of numerical aperture 0.6. The polarizing hologram 35 and thewavelength plate 436 are integrated as one body, and they are fixed to aholder 438 with the object lens 437. As shown in FIG. 10, the polarizinghologram 435 is fabricated by forming a hologram in a LiNb plate made ofa birefringence material with proton exchange. It transmitsextraordinary light and diffracts ordinary light. The light beam 32 ishandled as extraordinary light by the polarizing hologram 35, and ittransmits the polarizing hologram 35 without diffraction. The wavelengthplate 436 converts light of wavelength 650 nm from linear polarizationto generally circular polarization, but it does not change polarizationfor light of wavelength 780 nm. Thus, the light beam 32 is converted tocircular polarization. The object lens 437 is designed similarly to thecounterpart 408 in the ninth embodiment. As shown in FIG. 47, a centralportion 437 a of numerical aperture equal to or smaller than 0.45 hasminimum aberration for a disk plate thickness 0.9 mm while an outerportion 437 b of numerical aperture equal to or larger than 0.45 hasminimum aberration for a disk plate thickness 0.6 mm. The light beam 32is converged by the object lens 437 to form a light spot 439 on aninformation plane in the optical disk 10 of plate thickness 0.6 mm.

Next, the light 440 reflected by the optical disk 10 is condensed by theobject lens 437, is converted by the wavelength plate 436 from thecircular polarization to linear polarization having a polarizationdirection perpendicular to a polarization plane of the light beam 32 andenters the polarizing hologram 435. Because the reflected light 440enters the polarizing hologram 435 as ordinary light, it is diffracted.The diffraction divides the reflected light 440 into diffracted light442 a for detecting focus signal, diffracted light 442 b for detectingtracking signal and diffracted light 441 for light shielding. As shownin FIG. 45, the ring-like hologram diffracts the light 441 on a portionin correspondence to numerical aperture of 0.37 to 0.45 of the objectlens 437, so that the resultant light does not enter the photodetectors31 b and 31 c. The diffracted lights 442 a and 442 b are narrowed by thecondenser lens 434 to be received by the photodetectors 31 b and 31 c,respectively, and reproduction signals are detected by one or both ofthe photodetectors. Further, the photodetector 31 b detects a focuscontrol signal for making the object lens 37 follow the informationplane with spot size detection technique and the photodetector 31 cdetects a tracking control signal for tracking an information track withphase difference technique or push-pull technique.

On the other hand, as shown in FIG. 46, a second module 43 for CDcomprises a laser diode 43 a of wavelength 780 nm, a hologram 43 d forseparating light reflected from an optical disk to give spacial changeand photodetectors 43 a, 43 b for detecting the reflected light, andthey are integrated as one body. In FIG. 46, a part of a light beam 44of wavelength 780 nm emitted by the laser diode 43 a in the secondmodule 43 transmits the hologram 43 d and enters the wavelength filter33. Because the wavelength filter 33 reflects light of wavelength 780nm, the light beam 44 is reflected by the wavelength filter 33 and iscollimated by the condenser lens 34. The collimated light beam 44transmits the polarizing hologram 435 and the wavelength plate 436 toenter the object lens 437 of numerical aperture 0.6. The light beam 44is handled as extraordinary light by the polarizing hologram 435 and ittransmits the polarizing hologram 435 without diffraction. Because thewavelength plate 436 does not convert polarization direction of light ofwavelength 780 nm, the polarization plane of the light beam 44 ismaintained. Thus, the light beam 44 is focused by the object lens 437 toform a light spot 445 on an information plane in the optical disk 18 ofplate thickness 1.2 mm.

The light 46 reflected by the optical disk 18 is condensed by the objectlens 437 and transmits the wavelength plate 436 and the polarizinghologram 435. Because the wavelength plate 436 does not changepolarization direction for light of wavelength 780 mm, the reflectedlight 446 transmits the wavelength plate 436 as linear polarization,similarly to the light beam 44. Because the reflected light 46 entersthe polarizing hologram 435 as extraordinary light, it is notdiffracted. The light 46 transmitting the polarizing hologram 35 isnarrowed by the condenser lens 34 and is reflected by the wavelengthfilter 33 to enter the second module 43. The reflected light 46 enteringthe second module 43 is diffracted by the hologram 43 d to enter thephotodetectors 43 b and 43 c, and reproduction signals are detected byone or both of the photodetectors. Further, the photodetector 43 bdetects a focus control signal for making the object lens 437 follow theinformation plane with spot size detection technique and thephotodetector 43 c detects a tracking control signal for tracking aninformation track with phase difference technique or push-pulltechnique.

In the above-mentioned structure, when an optical disk of thickness 1.2mm such as CD is subjected to recording or reproduction, as shown inFIG. 47, the diffraction portion 435 a of the polarizing hologram 435diffracts the reflected light on a part in correspondence to numericalaperture from 0.37 to 0.45 of the object lens 437 to generate diffractedlight 441. Thus incidence thereof onto the photodetector is prevented.Then, the light 442 a, 442 b reflected by the diffraction portion 435 adoes not include components having large aberration. That is, only theabove-mentioned synthesized light is detected when a DVD is recorded orreproduced. Aberration of the region near the central axis of light beamof the object lens is small even when the design plate thickness isdeviated, and for numerical aperture of about 0.37 the aberration isabout 30 mλ rms when the light transmitting the central portion 437 a ofthe optimum design plate thickness 0.9 mm is focused on an optical diskof plate thickness of 0.6 mm. Then, the recording and reproducingperformance is not deteriorated. Therefore, recording and reproductioncan be possible for a DVD by using the above-mentioned light spot 411synthesized from light beams transmitting the outer portion 437 b andthe central portion 437 a of numerical aperture equal to or smaller than0.37.

When an optical disk 18 of thickness 1.2 mm such as CD is subjected torecording or reproduction, the range of numerical number 0.45 of theobject lens 437 is designed for optimum design plate thickness 0.9 mm,so that aberration of the light transmitting the object lens can besuppressed to a value about the same as that in a prior art device. Asshown in FIG. 48, the light beam transmitting the outer portion 437 b ofthe object lens 437 has large spherical aberration and it is diverged ina relatively wide range on an information plane in an optical disk 18,and the reflected light thereof is also diverged. Therefore, thereflected light transmitting the outer portion 437 b does not enter thephotodetector 22 generally. Because the polarizing hologram 435 does notdiffract both of the light beam 44 and the diffracted light 446, thenumerical aperture of the object lens becomes 0.45 for recording orreproduction of CD, and the numerical number of the same order as in aconventional CD drive can be obtained.

In this embodiment, a region numerical aperture 0.45 of the object lens437 provided for optimum design plate thickness 0.9 mm is explained.This region is a practical region because aberration on CD reproductionis suppressed to 40 mλ rms for 70% or more of the plate thickness ofoptical disk 18. In the above-mentioned example, the diffraction portion435 a corresponds to numerical aperture from 0.45 to 0.37 of the objectlens 437. The performance for a DVD can be secured for 70% or more ofthe numerical aperture 0.45 for a CD.

It is apparent from the above-mentioned explanation that according tothe ninth and tenth embodiments, light of first wavelength is shieldedor diffracted by the ring region having numerical aperture from NA3 toNA1 (0.7*NA1≦NA3<NA1), so that the numerical aperture of the centralportion of the object lens is set to NA3 practically for the light offirst wavelength, an information medium of disk plate thickness t1 canbe recorded or reproduced with small aberration.

Further, when an information medium of disk plate thickness t2 (t2>t3)is recorded or reproduced, the numerical number of the central portionof the object lens can be set to NA1 (>NA3). Because the numericalaperture is set optimally for plate thickness t3, and the aberration canbe made relatively small. As a result, an optical head using only oneobject lens can be provided wherein an information medium of disk platethickness t1 such as DVD can be recorded or reproduced with light of,for example, wavelength 650 nm while an information medium of disk platethickness t2 such as CD can be recorded or reproduced with light of, forexample, wavelength 780 nm.

Further, by using only one optical head, a DVD, a CD and a CD-R havingwavelength dependence can be recorded or reproduced, and compatibilitybetween DVD and all CDs can be secured.

Although the present invention has been described in connection with thepreferred embodiments thereof, it is to be noted that various changesand modifications are to be understood illustrative and not limiting.

1. An object lens for converging a light beam from a light source ontoeach of at least two types of optical information recording media havingdifferent thicknesses and being made of a transparent plate, said objectlens comprising: an inner region near a center axis of the light beamand an outer region far from the center axis, said outer region having aplane optimized to converge the light beam transmitting to said outerregion onto a first optical information recording medium among theoptical information recording media, and said inner region having aplane optimized to converge the light beam transmitting to said innerregion onto another optical information recording medium having a largerthickness than the first optical information recording medium; wherein aphase of the light beam transmitting to an innermost portion in theplane of said outer region is shifted relative to that of the light beamtransmitting to an outermost portion in the plane of said inner region,and wherein when the light beam is converged on the first opticalinformation recording medium, wave-front aberration satisfies acondition such that a total amplitude of aberration ≧20 mλ (rms) and afifth spherical aberration ≦20 mλ (rms).
 2. The object lens according toclaim 1, wherein when the light beam is converged on the first opticalinformation recording medium, the wave-front aberration satisfies acondition such that a seventh spherical aberration ≦30 mλ (rms).
 3. Theobject lens according to claim 1, wherein the plane of said inner regionis the plane optimized to converge the light beam transmitting to saidinner region onto the another optical information recording mediumhaving a smaller thickness than a second optical information recordingmedium among the optical information recording media.
 4. The object lensaccording to claim 3, wherein a direction of the shift of the phase ofthe light beam transmitting to the innermost portion of the plane ofsaid outer region is a forward direction.
 5. The object lens accordingto claim 1, wherein a numerical aperture, NA, of the plane of said innerregion and an NA of the entire aperture has a following relationship:0.7*NA of entire aperture ≦NA of inner region ≦0.8*NA of entire,aperture; wherein the phase shift of the light beam transmitting to theinnermost portion of the plane of said outer region to that of the lightbeam transmitting to the outermost portion of the plane of said innerregion has a value between 50 and 150 degrees.
 6. The object lensaccording to claim 1, wherein said object lens is optimized to convergethe light beam onto an information recording medium having a thicknessof the inner region equal to or smaller than t1*0.6, wherein t1 denotesa thickness of a plane of a second information recording medium amongthe optical information recording media.
 7. The object lens according toclaim 1, wherein the innermost portion of the plane of said outer regionand the outermost portion of the plane of said inner region construct asmooth line.
 8. An optical head comprising: light source operable togenerate a light beam; an object lens which converges the light beamgenerated from said light source onto each of at least two types ofoptical information recording media having different thicknesses andbeing made of a transparent plate; a photodetector which receives lightreflected from each of the optical information recording media toconvert it to an electric signal; wherein said object lens comprises aninner region near a center axis of the light beam and an outer regionfar from the center axis, said outer region having a plane optimized toconverge the light beam transmitting to said outer region onto a firstoptical information recording medium among the optical informationrecording media, and said inner region having a plane optimized toconverge the light beam transmitting to said inner region onto anotheroptical information recording medium having a larger thickness than thefirst optical information recording medium; wherein a phase of the lightbeam transmitting to an innermost portion in the plane of said outerregion is shifted relative to that of the light beam transmitting to anoutermost portion in the plane of said inner region; and wherein whenthe light beam is converged on the first optical information recordingmedium, wave-front aberration satisfies a condition such that a totalamplitude of aberration ≧20 mλ (rms) and a fifth spherical aberration≧20 mλ (rms).
 9. The optical head according to claim 8, wherein when thelight beam is converged on the first optical information recordingmedium, the wave-front aberration satisfies a condition such that aseventh spherical aberration ≦30 mλ (rms).
 10. The optical headaccording to claim 8, wherein the plane of said inner region is theplane optimized to converge the light beam transmitting to said innerregion onto the another optical information recording medium having asmaller thickness than a second optical information recording mediumamong the optical information recording media.
 11. The optical headaccording to claim 10, wherein a direction of the shift of the phase ofthe light beam transmitting to the innermost portion of the plane ofsaid outer region is a forward direction.
 12. The optical head accordingto claim 8, wherein a numerical aperture, NA, of the plane of said innerregion and an NA of the entire aperture has a following relationship:0.7*NA of entire aperture ≦NA of inner region ≦0.8*NA of entireaperture; wherein the phase shift of the light beam transmitting to theinnermost portion of the plane of said outer region to that of the lightbeam transmitting to the outermost portion of the plane of said innerregion has a value between 50 and 150 degrees.
 13. The optical headaccording to claim 8, wherein said object lens is optimized to convergethe light beam onto an information recording medium having a thicknessof the inner region equal to or smaller than t1*0.6, wherein t1 denotesa thickness of a plane of a second information recording medium amongthe optical information recording media.
 14. The optical head accordingto claim 8, wherein the innermost portion of the plane of said outerregion and the outermost portion of the plane of said inner regionconstruct a smooth line.
 15. The optical head according to claim 8,wherein at least two of said photodetector are provided for the at leasttwo optical information recording media of different thicknesses.
 16. Anoptical information recording and reproducing apparatus comprising: alight source operable to generate a light beam; an object lens whichconverges the light beam generated from said light source onto each ofat least two types of optical information recording media havingdifferent thicknesses and being made of a transparent plate; aphotodetector which receives light reflected from each of the opticalinformation recording media to convert it to an electric signal; asignal processor which distinguishes the type of optical informationrecording medium and reads information selectively from the electricsignal; wherein said object lens comprises an inner region near a centeraxis of the light beam and an outer region far from the center axis,said outer region having a plane optimized to converge the light beamtransmitting to said outer region onto a first optical informationrecording medium among the optical information recording media, and saidinner region having a plane optimized to converge the light beamtransmitting to said inner region onto another optical informationrecording medium having a larger thickness than the first opticalinformation recording medium; wherein a phase of the light beamtransmitting to an innermost portion in the plane of said outer regionis shifted relative to that of the light beam transmitting to anoutermost portion in the plane of said inner region; and wherein whenthe light beam is converged on the first optical information recordingmedium, wave-front aberration satisfies a condition such that a totalamplitude of aberration ≧20 mλ (rms) and a fifth spherical aberration≦20 mλ (rms).
 17. The apparatus according to claim 16, wherein when thelight beam is converged on the first optical information recordingmedium, the wave-front aberration satisfies a condition such that aseventh spherical aberration ≦30 mλ (rms).
 18. The apparatus accordingto claim 16, wherein the plane of said inner region is the planeoptimized to converge the light beam transmitting to said inner regiononto the another optical information recording medium having a smallerthickness than a second optical information recording medium among theoptical information recording media.
 19. The apparatus according toclaim 18, wherein a direction of the shift of the phase of the lightbeam transmitting to the innermost portion of the plane of said outerregion is a forward direction.
 20. The apparatus according to claim 16,wherein a numerical aperture, NA, of the plane of said inner region andan NA of the entire aperture has a following relationship: 0.7*NA ofentire aperture ≦NA of inner region ≦0.8*NA of entire aperture; whereinthe phase shift of the light beam transmitting to the innermost portionof the plane of said outer region to that of the light beam transmittingto the outermost portion of the plane of said inner region has a valuebetween 50 and 150 degrees.
 21. The apparatus according to claim 16,wherein said object lens is optimized to converge the light beam onto aninformation recording medium having a thickness of the inner regionequal to or smaller than t1*0.6, wherein t1 denotes a thickness of aplane of a second information recording medium among the opticalinformation recording media.
 22. The apparatus according to claim 16,wherein the innermost portion of the plane of said outer region and theoutermost portion of the plane of said inner region construct a smoothline.
 23. The apparatus according to claim 16, wherein at least two ofsaid photodetector are provided for the at least two optical informationrecording media of different thicknesses.